In order to determine the incidence and causes of death during the first 100 days after BMT (early deaths) in a pediatric population we have examined data reported in the AIEOP BMT Registry. Up to July 1990, data on 486 children who underwent allogeneic (180) or autologous (306) BMT were evaluable. The children had acute lymphoblastic leukemia (148 cases), acute non-lymphoblastic leukemia (127 cases), neuroblastoma (82 cases), chronic myelogenous leukemia (15 cases), aplastic anemia (nine cases), solid tumors, lymphoma, immunodeficiency or storage diseases. The overall survival is 55% for allogeneic HLA matched and 38% for autologous transplants at 5 years, 24% for HLA mismatched graft at 2 years. Out of the 486 children, 70 (14%) died during the first 100 days after BMT: 33/306 (11%) after autologous BMT, 24/150 (16%) after allogeneic matched BMT and 13/30 (43%) after mismatched BMT. Causes of early death were as follows: disease progression: 12 children (10/306 after autologous and 2/180 after allogeneic BMT); infection: 12 children (five after autologous and seven after allogeneic BMT); interstitial pneumonitis: 21 children (seven after autologous and 14 after allogeneic BMT); cardiac failure: five children (four after autologous BMT); veno-occlusive disease: eight children (three after autologous, five after allogeneic BMT); acute renal failure: three children (one after autologous and two after allogeneic BMT); multiple organ failure: two cases (one after autologous BMT); cerebral hemorrhage: three children (one after autologous BMT); hypertension: one child; acute GVHD: three children (12% of early deaths after allogeneic BMT).