We assessed the prognostic importance of the platelet count 100 days post transplant of 107 consecutive patients receiving ablative allogeneic bone marrow transplant (BMT) between 7/96 and 12/00 who survived at least 100 days. Diagnoses included AML (n=36), chronic myelogenous leukemia (n=27), NHL (n=14), ALL (n=16), MDS (n=9), aplastic anemia (n=3), and one Hodgkin's disease and myelofibrosis each. In total, 64% were in remission or in chronic phase or had aplastic anemia (good risk), and 36% had active disease at the time of transplant (bad risk). In all, 70% were matched sibling transplants and 30% were matched unrelated donor transplants. The mean follow-up for the patients remaining alive is 48 months. Survival was powerfully influenced by the 100-day platelet count: 4-year survival was 19% for patients with a platelet count <30 x 10(9)/l; 41% for patients with a platelet count of 30-50; and 72% for those with a platelet count >50 (P<0.001; log-rank test). In a multivariable analysis, the most powerful risk factors for mortality after allogeneic BMT were low 100-day platelet count (P<0.001) and bad risk disease (P=0.009). We conclude that the platelet count 100 days post transplant is a powerful predictor of overall survival.