VIRGINIA BEACH -- A 4-year-old boy has returned to his home here, three months after an experimental operation to treat his leukemia with umbilical cord blood collected during the birth of his sister.

Michael Sancilio left Johns Hopkins Hospital in Baltimore Saturday, hospital officials said.

"Although doctors are reluctant to say Michael is cured, Michael appears to have defeated a rare form of leukemia for which previously existed no known cure," said a statement from Virginia Beach General Hospital. Doctors there collected so-called stem cells from his sister's umbilical cord.

Michael has "another chance at life" with a new blood type and new bone marrow, the statement issued yesterday said.

The boy is the first patient to undergo the transplant to treat leukemia, a cancer of the blood and bone marrow.

Umbilical blood, usually discarded at birth, is rich in stem cells. Stem cells are young cells that multiply rapidly to form bone marrow.

Michael's chronic myelogenous leukemia was diagnosed eight months ago. His mother was seven months pregnant when doctors told her Michael had less than one year to live.

Christina Grace Sancilio was born May 1. Within a few days, tests showed the siblings were a good match, although they had different blood types, doctors said.

After intensive chemotherapy, doctors at Johns Hopkins transplanted the umbilical blood into the boy. Within one month, Michael began producing healthy bone marrow, doctors said.

Doctors will not know for about two years whether the boy is cured. Christina is healthy, doctors said.

Doctors in France performed the first cord blood transplant in 1988 to treat a rare form of anemia.