One hundred fifty-six episodes of fever occurred in 102 children during the first 100 days after bone marrow transplantation (BMT) performed at a single institution: fever of undetermined origin (FUO), 40.3%; septicemia, 7.1%; pneumonia, 19.2%; other infections, 33.4% of cases. The overall incidence of mortality was 22.6% and of mortality due to infections 17.4%. All FUO episodes resolved. Pneumonia was the major cause of death; 60% of recipients who developed pneumonia died, accounting for 90% of deaths attributable to febrile complications. Interstitial pneumonia, occurred rarely, in 3.9% of all febrile episodes. The Cox model showed that the presence of graft-versus-host disease (GVHD) was related to an approximately ninefold increase in the risk of a first episode of FUO (P value .03). The risk of developing pneumonia was fourfold greater in children who received a transplant from a matched unrelated donor or a mismatched family donor (P value .01). Developments in diagnostic tools are needed to diagnose febrile episodes earlier and more precisely with the aim of reducing early mortality after BMT.