For years, Jon Hanson was haunted by questions about his past: Who were his parents? Why had he been removed from his home as a child and put into the foster care system? Were his parents still living--and, if so, where?

Eventually though, Hanson put the past behind him, focusing instead on the challenges of making a life for himself and his growing family.

"You have to move on," said Hanson, who is 30 and lives in the Calvert County town of Lusby. "The past is the past. You have to go on with life."

But suddenly, last month, Hanson's past became his future--or, more specifically, his little girl's future.

In November, Hanson's 7-year-old daughter, Morgan Elise Hanson, was diagnosed with leukemia and will soon need a bone-marrow transplant to save her life. Unfortunately, all immediate family members--often the most likely compatible donors--have been ruled as incompatible. That means that doctors at Children's Hospital in Washington, where Morgan is hospitalized, will turn to a national registry to find a compatible donor, a process that could take months.

Hanson, though, is pursuing another route that--although a long shot, according to one doctor--could help Morgan find a donor sooner. Hanson hopes that one of his long-lost relatives might be compatible with Morgan--but he needs to find his family now.

"I need them," Hanson said. "I need them in this particular situation. It's not about me. It's about my daughter. It's about giving her an opportunity to live."

It's about a little girl who, her mother said, "loves horses and horseback riding, takes tap lessons," has "a beagle named Lucky that she loves to death . . . a child who needs a lot of other children around.

"Now," her mother said, "she's in a filtered room. . . . It's every parent's nightmare."

The nightmare went like this: For Halloween, the Hansons were watching Morgan prance around dressed as a ballerina; for Christmas, they were putting up a little artificial tree in her hospital room. In between, on Nov. 18, after Morgan remained lethargic after an ear infection cleared up, she was diagnosed with leukemia. Her parents were told to rush her to Children's, and she has remained there since.

Meanwhile, her father has renewed his search for his birth parents, who, if they are still alive, may actually be relatively close at hand, because Hanson was born in Prince George's County. The obstacle, though, is cutting through the legal roadblocks that often prevent adoptees from finding their birth parents. That's what originally forced Hanson to put his search on hold.

"It gets into a financial thing, and time--time goes by so quick," said Hanson, who has worked mostly as a boat mechanic, but now is unemployed. "You get frustrated. You don't really want to go through all the red tape."

Hanson's main clue comes from a birth certificate he once saw: It revealed that he was born March 15, 1969, at Prince George's Hospital--and that his given name was Johnnie Ray Peebles.

The remaining information he has is somewhat murky. Hanson said he knows that he was "in and out of foster care" for much of his childhood; he also has memories of his mother, whose first name he believes was Diane or Diana. "I remember being picked up by her on weekends," he said.

Eventually he came to live at a Baltimore orphanage from age 10 to 13, at which time he was adopted. He uses the name of those adoptive parents as his own.

"If it wasn't for them," Hanson said, "I probably would be a statistic on the street."

Hanson went on to attend Northern High School in Calvert County and was able to redirect his life by participating in numerous sports. He refrained from seeking his birth parents out of fear--that he would hurt his adoptive parents and that his birth parents would reject him. But inside, he never gave up hoping that he would find the truth someday.

"It's just the feeling of wanting to know where you come from--to know your roots," Hanson said. "It's a void in your life. It sits in the back of your mind."

Hanson served a couple of years in the Navy after high school, got married, had his first child, and divorced quickly. He met his second wife, Jeanine, in 1991 while he was tending bar at a restaurant in Anne Arundel County. On Dec. 22, 1992, his wife, who now delivers newspapers for The Washington Post, gave birth to Morgan--the first of their three children.

Seven years later, Morgan is suffering from acute myelogenous leukemia, a form of the blood cancer that's far less responsive to therapy than other strains, according to Gregory Reaman, who serves as both the chairman of the Division of Hematology/Oncology at Children's as well as the executive director of the hospital's Center for Cancer and Blood Disorders. The cure rate is about 45 percent, Reaman said, compared with the 80 percent cure rate for those suffering from the more common acute lymphocytic leukemia.

The form of treatment, Reaman said, is "very intensive chemotherapy . . . to basically destroy normal bone marrow as well as leukemia." Once the cancer is in remission, the next step is for the patient to undergo a transplant of healthy bone marrow from a donor with matching tissue-type. Tissue-type, Reaman said, is determined by human leukocyte antigens, "structures on the surface of cells that are responsible for making them unique." There are six particular antigens that should be compatible in a donor and recipient--otherwise it's more likely transplanted bone marrow will be rejected.

Because a person's genetic makeup determines the types of antigens he or she has, siblings can make good donors for one another, according to Reaman. Parents are generally less compatible, but sometimes they can be used as donors for their children. It's even less likely that grandparents could be good donors for their grandchildren.

"I think the possibility is probably in the area of one in ten thousand or one in a hundred thousand," Reaman said. "It's an unlikely possibility."

That's why doctors are advising the Hansons that it doesn't make sense to spend all the time needed to track down long-lost relatives, and that it's best to just find a donor through a national registry.

"Time is sort of the essence, but our hope would be we could transplant her within three to six months of remission," Reaman said. "It usually takes that long to identify a donor through the registry. . . . Tracking someone's parents through an adoption situation would probably take longer."

The only good news for Hanson is that recently lawmakers have made it easier for adoptees to track down their birth parents. In Maryland, for example, they need to go to the Department of Social Services in the county where they were born or currently live, and to see a confidential intermediary--a social worker trained to assist in such matters--to help arrange a reunion.

The catch is that the birth parent needs to be found--and any meeting needs to be consensual.

"Reunions would not be happening if you did not have two consenting parties," said Rebecca Watson, a social work associate for the Calvert County Department of Social Services.

In the meantime, Morgan is undergoing phases of chemotherapy treatment at the hospital, where she celebrated her seventh birthday Wednesday and where she spent Christmas Day.

Ask Morgan--who has been visited by everyone from figure skater Tara Lipinski to first lady Hillary Rodham Clinton--how she feels, and she'll tell you: "Good."

Ask Hanson about his daughter, who has lost all her pretty blond hair, and he'll tell you: "To me, she looks like a very sick girl. She's not the girl we brought in here."

And that's why, long shot or not, he's pressing to find the long-lost relatives who may help save his little girl.

"It's a chance I have to take," he said. "I have to take it. . . . I'd rather try and know it worked, than not to have tried at all."

CAPTION: "I need them," says Jon Hanson of his birth parents. His daughter, Morgan, 7, needs a bone marrow transplant, and Hanson hopes for a match from his long-lost family.

CAPTION: Morgan Hanson is undergoing preliminary treatment for leukemia and awaiting news of a bone marrow donor at Children's Hospital.