The purpose of the present study was to analyze the characteristics of infectious complications occurring during the first 100 days after bone marrow transplantation (BMT) in a cohort of 123 pediatric patients with hematological malignancies (n = 73), solid tumors (n = 32) and nonmalignant disorders (n = 18). Fifty-eight patients received allogeneic grafts, and 65 patients an autologous transplant. Fever developed in 107 (87%) children; 82% of infectious complications occurred during the neutropenic period. Documented infection developed in 33 (31%) patients, while 74 (69%) patients had possible infection (i.e. fever of unknown origin). The incidence of bacteremia was 21%, and gram-positive cocci were the predominant pathogens; non-bacteremic microbiologically documented infection developed in 6% of patients; clinically evident infection developed in 4% of subjects. The incidence of primary febrile episodes was not significantly different between autologous and allogeneic BMT (86% vs 88%); nor did the median number of days to the onset of fever (5 days in both groups) or the median duration of fever (5 days in both groups) differ. In contrast, the frequency of secondary febrile episodes was significantly higher (P = 0.0001) in allogeneic BMT recipients (40%) than in autologous recipients (15%). The mortality rate due to infections was 2/36 (5%) for matched sibling donor BMT, and 1/13 (8%) for matched unrelated donor BMT. No deaths occurred in the 65 patients who were autografted. Invasive fungal infections accounted for 2 of the 3 infectious deaths. In conclusion, the majority of children undergoing BMT experienced at least one infectious episode; allogeneic BMT recipients were at high risk of developing secondary febrile episodes, but the overall mortality rate due to infection in the first 100 days after transplantation was low.