We performed allogeneic bone-marrow transplantation of normal cells in an eight-year-old patient with Type 3 Gaucher's disease in an attempt to alter his progressive deterioration. The procedure resulted in complete engraftment of the enzymatically normal donor cells. Donor monocyte precursors were present in the circulation of the recipient by 28 days after transplantation. Post-transplantation beta-glucocerebrosidase activity in mononuclear white cells in peripheral blood exceeded normal levels. Despite these early results no change in Gaucher's-cell infiltration of the bone marrow was noted for at least 111 days, and Gaucher's cells persisted in the marrow for at least 274 days. The plasma glucocerebroside concentration ultimately returned to normal. During the post-transplantation course, until the patient died from an episode of sepsis, there was no important change in his clinical status. However, this case demonstrates that the enzymatic abnormality in hematopoietic cells in Gaucher's disease is correctable by bone-marrow transplantation, leading to normalization of plasma levels of glucocerebroside. We observed the long survival of Gaucher's cells, which was consistent with the lack of clinical improvement. Intervention might have been more effective earlier in the course of the disease.