Surgeons in Southern California have performed the first human bladder transplant, introducing a new, potentially life-changing procedure for people with debilitating bladder conditions.

The operation was performed earlier this month by a pair of surgeons from the University of California, Los Angeles, and the University of Southern California on a 41-year-old man who had lost much of his bladder capacity from treatments for a rare form of bladder cancer.

“I was a ticking time bomb,” the patient, Oscar Larrainzar, said on Thursday during a follow-up appointment with his doctors. “But now I have hope.”

The doctors plan to perform bladder transplants in four more patients as part of a clinical trial to get a sense of outcomes like bladder capacity and graft complications before pursuing a larger trial to expand its use.

Dr. Inderbir Gill, who performed the surgery along with Dr. Nima Nassiri, called it “the realization of a dream” for treating thousands of patients with crippling pelvic pain, inflammation and recurrent infections.

“There is no question: A potential door has been opened for these people that did not exist earlier,” said Dr. Gill, the chairman of the urology department at U.S.C.

Pushing the Envelope

Until now, most patients who undergo a bladder removal have a portion of their intestine repurposed to help them pass urine. Some receive an ileal conduit, which empties urine into a bag outside the abdomen, while others are given a so-called neobladder, or a pouch tucked inside the body that attaches to the urethra and allows patients to urinate more traditionally.

But bowel tissue, riddled with bacteria, is “inherently contaminated,” Dr. Gill said, and introducing it to the “inherently sterile” urinary tract leads to complications in up to 80 percent of patients, ranging from electrolyte imbalances to a slow reduction in kidney function. The loss of the intestinal segment can also cause new digestive issues.

Dr. Despoina Daskalaki, a transplant surgeon at Tufts Medical Center who was not involved in the new procedure, said advances in transplant medicine (from critical life-sustaining organs, like hearts and livers, to other body parts, like faces, hands, uteri and penises) had led doctors to start “pushing the envelope.”

“They’re asking: ‘Why do we have to put up with all the complications? Why don’t we try and give this person a new bladder?’” Dr. Daskalaki said.

In late 2020, Dr. Nassiri was in his fourth year of residency at the University of Southern California when he and Dr. Gill sat down in the hospital cafeteria to begin brainstorming approaches. After Dr. Nassiri began a fellowship on kidney transplantation at U.C.L.A., the two surgeons continued working together across institutions to test both robotic and manual techniques, practicing first on pigs, then human cadavers, and finally, human research donors who no longer had brain activity but maintained a heartbeat.

One of the challenges of transplanting a bladder was the complex vascular infrastructure. The surgeons needed to operate deep inside the pelvis of the donor to capture and preserve a rich supply of blood vessels so the organ could thrive inside the recipient.

“When we’re removing a bladder because of cancer, we basically just cut them. We do it in less than an hour on a near-daily basis,” Dr. Gill said. “For a bladder donation, that is a significantly higher order of technical intensity.”

The surgeons also chose to conjoin the right and left arteries — as well as the right and left veins — while the organ was on ice, so that only two connections were needed in the recipient, rather than four.

When their strategy was perfected in 2023, the two drew up plans for a clinical trial, which eventually would bring the world’s first recipient: Oscar.

An Ideal First Candidate

When Mr. Larrainzar walked into Dr. Nassiri’s clinic in April 2024, Dr. Nassiri recognized him. Almost four years earlier, Mr. Larrainzar, a husband and father of four, had been navigating end-stage kidney disease and renal cancer, and Dr. Nassiri helped remove both of his kidneys.

But Mr. Larrainzar had also survived urachal adenocarcinoma, a rare type of bladder cancer, and a surgery to resect the bladder tumor had left him “without much of a bladder at all,” Dr. Nassiri said. A normal bladder can hold more than 300 cubic centimeters of fluid; Mr. Larrainzar’s could hold 30.

Now, years of dialysis had begun to fail; fluid was building up inside his body. And with so much scarring in the abdominal region, it would have been difficult to find enough usable length of bowel to pursue another option.

“He showed up serendipitously,” Dr. Nassiri said, “but he was kind of an ideal first candidate for this.”

On a Saturday night earlier this month, Dr. Nassiri received a call about a potential bladder match for Mr. Larrainzar. He and Dr. Gill drove straight to the headquarters of OneLegacy, an organ procurement organization, in Azusa, Calif., and joined a team of seven surgeons working overnight to recover an array of organs from a donor.

The two brought the kidney and bladder to U.C.L.A., then stopped home for a shower, breakfast and a short nap. They completed the eight-hour surgery to give Mr. Larrainzar a new bladder and kidney later that day.

Dr. Nassiri said that kidney transplants can sometimes take up to a week to process urine, but when the kidney and bladder were connected inside Mr. Larrainzar, there was a great connection — “immediate output” — and his creatinine level, which measures kidney function, started to improve immediately. Mr. Larrainzar has already lost 20 pounds of fluid weight since the surgery.

The biggest risks of organ transplantation are the body’s potential rejection of the organ and the side effects caused by the mandatory immune-suppressing drugs given to prevent organ rejection. That is why, for Dr. Rachel Forbes, a transplant surgeon at Vanderbilt University Medical Center who was not involved in the procedure, the excitement is more tempered.

“It’s obviously a technical advance,” she said, but “we already have existing options for people without bladders, and without the downside of requiring immunosuppression.” Unless a patient is — like Mr. Larrainzar — going to be on those medications anyway, “I would be a little bit nervous that you would be exchanging some complications for others,” she said.

A new bladder transplant also does not have nerve connections in the recipient, so while it works well as a storage organ, doctors did not know whether Mr. Larrainzar would ever be able to sense a full bladder, let alone hold and empty it naturally. They spoke about catheters, abdomen maneuvers and eventually developing an on-demand bladder stimulator to help with the release.

But at a follow-up appointment on Thursday morning — just two days after Mr. Larrainzar was discharged from the hospital — Dr. Nassiri removed the catheter and gave him fluids, and Mr. Larrainzar immediately felt that he could urinate.

Dr. Nassiri called it a miracle, then phoned Dr. Gill, who was in a U.S.C. operating room, and exclaimed two words: “He peed!”

“No way! What the hell?” Dr. Gill said. “My jaw is on the floor.”

After finishing the surgery, Dr. Gill drove straight to U.C.L.A. and watched Mr. Larrainzar do it again.

“Of course, this is very, very early. Let’s see how everything goes,” Dr. Gill cautioned. “But it’s the first time he has been able to pee in seven years. For all of us, this is huge.”

Mr. Larrainzar, exhausted, smiled, and Dr. Nassiri brought him a bottle of mineral water to celebrate.