Recent years have witnessed a reduction in the frequency of infectious complications both in allogeneic and in autologous bone marrow transplantation (BMT). With a frequency of 30-40 percent, septicaemia is the predominant problem during the aplastic phase. Owing to the introduction of prophylaxis with quinolones, there has been a shift in the aetiological spectrum toward Gram-positive infections, and Gram-negative infections now account for less than 10 percent of all cases. In turn, this has been accompanied by reduced mortality. Early institution of treatment based on viraemia diagnosis has radically reduced the substantial morbidity and mortality formerly due to cytomegalovirus infection during the first three months after BMT. Antibiotic prophylactic treatment for Pneumocystis carinii infection during the first year, and against pneumococcal infections thereafter, has also been highly successful. The remaining problems are the diagnosis and treatment of invasive fungal infections and the increased susceptibility to infection among those who develop graft-versus-host disease.