Mr. Straus. (Courtesy Marc Strauss Gallery)

On a Friday last month, Marc Straus was sitting in the office of a new gallery on the Lower East Side, recalling the decades that he spent working as an oncologist, and the optimism he felt for the students he taught in the field.

“I thought, damn, if I train you, you’re going to go out and be a rigorous son of a bitch,” Mr. Straus said. “You’re going to keep reading your whole career, and you’re not going to be lazy.” That sounds like tough talk, but the mostly retired doctor—he sees patients on occasion—was speaking calmly and happily, and he was grinning.

“It turned out usually not to be true,” Mr. Straus said, laughing out of exasperation. “Five years later, very few of them kept up and had the extra rigor to keep reading.” He was setting up a comparison. “Livia and I never stopped looking at new art,” he said, referring to his wife, “which in some ways I think may be more important than owning the art.”

That is a peculiar bit of wisdom coming from a man who recently elected to become a contemporary art dealer, after collecting art for more than four decades. But Mr. Straus is not an ordinary art dealer. He has run a medical practice with 150 employees and 70,000 patients, started medical software companies, written important texts on lung cancer and served on museum boards. He is also a published poet. And an architect; he helped design the roughly 3,000-square-foot gallery in which we were sitting. Structurally speaking, it easily qualifies as one of the most ambitious commercial spaces on the Lower East Side.

Walking The Observer through the four-story building at 299 Grand Street, Mr. Straus pointed out the hickory floors—“the hardest wood we know”—that date back more than 300 years, made of wood from a barn in Kentucky, and metal grates on each floor that, when opened, allow 10-foot-long paintings to be raised out of a basement storage area.

That basement storage area has pressed-tin ceilings like those one might find at a tony new bar in the neighborhood, and the second-floor galleries feature double-height ceilings, the result of Mr. Straus’s gutting a stretch of the third floor. It is, by any measure, an impressive undertaking in a burgeoning art district better known for scrappy and modest storefront galleries with uneven cement floors.

Not surprisingly, some in the art world have expressed bafflement at Mr. Straus’s decision to take on the role of dealer, and to do so in such grand fashion, essentially going into competition with some of the people he has supported as a buyer.

One area dealer called the endeavor a “vanity project,” a phrase that another used to describe the museum that the Strauses founded in Peekskill, N.Y., the Hudson Valley Center for Contemporary Art (HVCCA). “People are skeptical,” one artist told us.

That skepticism has not hurt business, according to Mr. Straus. “We pretty much sold out our first three shows,” he said. “All three.” (The gallery operated out of a temporary space on Delancey while the massive renovation program was underway.) Are his collector-friends buying? “Nobody’s going to write a check for $10,000 just to help me out.” Fair enough.

Visitor traffic also appears to be strong. One Sunday last month, the gallery’s inaugural opening reception was packed. One display was of sculptures and wall pieces made out of painted slices of cardboard by Birgit Brenner, a midcareer German artist who shows with Eigen+Art, a major gallery with branches in Leipzig and Berlin.

“I met the owner of Eigen+Art 21 years ago, and I think we have been friends since then,” Mr. Straus said. “He said to me, ‘O.K., I collaborate with Zwirner, Pace and Hauser & Wirth’”—listing three galleries many would consider among the top five in New York—“and now Marc Straus,’ and I think, ‘Wow. I’ve admired him since the beginning, when he had no money. I admired his integrity, his support of artists.’ And I think, ‘Wow, what an honor.’”

The other show was by Zlatan Vehabovic, a young Croatian artist who paints dark, moody, sometimes mysterious figurative scenes. It’s his first solo show outside of his home country; the Strauses came across his work during one of their extensive, studio-visit-filled tours of Europe.

Mr. Straus’s transition from collector to dealer is unusual, though not unprecedented. Collectors Peggy Guggenheim and, more recently, Robert Mnuchin are rare exceptions, and a handful of high-flying collectors—the British advertising magnate Charles Saatchi and the Warhol-obsessed Mugrabi family, to name two—buy and sell with the regularity of gallerists.

Vouching for his instincts are Mr. Straus’s early acquisitions of work by artists like Gilbert & George, Kiki Smith and Robert Gober, and the fact that various regional museums have hosted his collection over the years.

Ultimately, the gallery will live or die by the art it shows and Mr. Straus’s ability to sell it to collectors and, in a different sense, to curators and critics—in other words, to put his roster through the art world’s multitiered endorsement process. To help with that, he hired away the young curator James Cope from the Dallas-based Goss-Michael Foundation—as in, George Michael and his partner Kenny Goss—where, for six years, Mr. Cope had helped organize shows of major contemporary artists, like Jim Lambie, Tracey Emin and Michael Craig-Martin.

Why trade a secure foundation job to work as gallery director for an untested art dealer, we asked Mr. Cope? He was sitting next to Mr. Straus, in horn-rimmed glasses, a tidy haircut and a neat beard. “When you work with a gallery where you really, truly believe in the artists—several of the artists I am very good friends with—it’s a lot more exciting,” he answered with the easy equanimity of his employer. “The relationships are built to last.”

Being a collector has given Mr. Straus insights into his new role: do not overprice the work and “honor the person who comes in and writes a check.” He explained: “When a dealer says—and this has happened, God knows, thousands of times—‘This is cheap,’ I almost cringe. They’re asking $20,000 for a kid wet behind the ears, and it’s after-tax money. The average pediatrician is making $80,000 a year. What are they talking about? When I walk into a show, I know how it should be priced.”

Messrs. Straus and Cope have debuted with a lineup of 14 artists—none stolen from other gallerists, the former emphasized. Many have never had a solo show, though more than half have had been in at least one exhibition at the HVCCA, where Livia Straus first met Michael Brown, who showed at Yvon Lambert in New York, before the esteemed Paris dealer closed his gallery last year. The Strauses were the first collectors to buy one of his pieces. Now Mr. Straus, who helped introduce Mr. Brown to Yvon Lambert, shows the artist in New York. “I didn’t have to think twice,” Mr. Brown told us, of joining the gallery.

Mr. Brown showed at the temporary space in October, and when Mr. Straus walked in he told us that he said, “‘Holy shit, this work is so good.’ I never thought for one moment I would ever represent him. Now my job is to get him a great gallery in Europe, and I know it’s going to happen. Now my job is to give a larger voice to his work.”

It’s not just Mr. Brown who is devoted to him. “The depth of experience [Marc] brings with him is what sets him apart from your average dealer,” painter John Newsom, whom the couple has known for a decade, told us in an email. “He knows artists. He knows what they need, and what makes them tick. Marc expects the best, a type of ‘exceptionalism.’”

Mr. Straus has roughed it like some of today’s Lower East Side dealers before, albeit only for about a year, as a partner in the East Village gallery called Piezo Electric in the mid-1980s, which showed talent like Richard Hambleton, sculptor Michael Gonzalez and geometric abstractionist Richard Kalina. “It was very successful, and when I say successful, I mean we didn’t lose money,” he said.

As a poet—Mr. Straus has been writing for 20 years—he has lectured and taught at universities and conducted workshops. Still he gets the occasional doubter. He told us about “some snobby guy,” another poet, who once claimed that, “well, Marc Straus is only here because he’s a doctor.”

“Nobody is going to ask me, when they walk into this gallery, if this something I’m just doing as a hobby.”

He continued, “This is a business, and I have to say—this could be misunderstood—it’s not a good business.” Every month, he explained, you have to reinvent yourself. You don’t know if you’ll be able to sell an artist’s work, or what that artist will do next year. But you do it because you believe in the artists you represent. “You have to be crazy,” he said, “to go into this business.”

arusseth@observer.com