Before treatment of 181 patients with bone marrow transplantation (BMT) for leukemia, severe aplastic anemia, or metabolic disorders, the oral condition was examined clinically and roentgenologically. Fifty-three patients (29%) had chronic dental infections (osteitis) that needed treatment before BMT. In 10 of 181 cases (6%), BMT was postponed because of oral infections. Septicemia during the neutropenic phase was caused by oral microorganisms (alpha streptococci) in 24 of 59 (41%) patients with microbiologically proven septicemia. Septicemia with alpha streptococci was associated with graft-versus-host disease prophylaxis with methotrexate and subsequent increased frequency of oral ulcerations. No difference was observed in the frequency of reactivation of latent herpes simplex virus infection between different graft-versus-host disease prophylaxis regimens. Reactivation was more frequent in patients conditioned with total body irradiation than in patients conditioned without total body irradiation. Antiviral prophylaxis, with subsequent decreased frequency of oral herpes simplex reactivation, appeared to contribute to a low frequency of septicemia with alpha streptococci.