It was supposed to be her last appointment. Stricken with leukemia at age 25, Devendar Mata had been in remission for five years and hoped her doctor would finally pronounce her cured.

That’s not what happened. The physician noticed that her white blood cell count was slightly low. He figured it was a minor infection, nothing to worry about. Ms. Mata, a software designer and recent Indian immigrant, wasn’t so sure. Three weeks later, her fears were confirmed: The cancer was back.

Nine hundred miles away from Ms. Mata’s home in St. Louis, Peter McMahon had just joined the New York Fire Department, after losing his brother Robert, also a firefighter, on September 11, 2001. Mr. McMahon donated blood at the fire academy and, like most of the city’s firefighters, agreed to join the National Marrow Donor Program Registry.

That decision saved Ms. Mata’s life.

The two will meet for the first time today, as the New York Blood Center honors the Fire Department, whose members form the single largest group in the agency’s bone-marrow donation program. More than 8,000 firefighters have joined the registry since 1990, and 64 have been matches.

“It’s not a big deal to them because they save lives every day,” a spokeswoman for the Blood Center, Linda Levi, said. “But to us and the people they save, it’s a very big deal.”

To join the registry, donors contribute an extra tube of blood. If a match is found and the donor agrees, both parties check into hospitals in their respective locations. Stem cells are drawn from the donor’s bloodstream or bone marrow and hand-transported in a cooler to the recipient.

A firefighter at Ladder Company 153 in Brooklyn, Greg Hansen, had been on the job two years when he learned that he was a match.

The son of a retired fire captain, Mr. Hansen, who lives at Staten Island, readily agreed to donate marrow. But his identity was kept secret from his recipient, Melanie Harman, as hers was from him.

A West Virginia teaching assistant and mother of three, Ms. Harman, 37, was first diagnosed with Hodgkin’s lymphoma in 1999. Her chemotherapy was successful, but four years later, her doctor found acute myelogenous leukemia, the same disease that plagued Ms. Mata. AML, as it’s known, is caused by a defect in a patient’s bone-marrow stem cells that causes the marrow to produce unhealthy blood. Patients are often tired, short of breath, easily injured, and prone to infections.

For Ms. Harman, a bone-marrow transplant was the best possible hope. Her twin sister, an obvious first thought, was disqualified for fear that the marrow would be too similar, and thus produce blood cells indistinguishable from her own. So her doctor consulted the national registry and found Mr. Hansen.

Ms. Harman is now cancer-free. “I’m just so appreciative,” she said. “Without him I probably wouldn’t be here.”