BACKGROUND: Patients receiving bone marrow transplantation (BMT) are highly susceptible to infection. This study aims to analyze the frequencies, types and distributions of the organisms causing infectious complications following BMT. METHODS: We retrospectively analyzed infectious complications of 76 bone marrow transplant patients treated at Chonnam National University Hospital during the period 1992~1999. RESULTS: The patient group consisted of 52 allogeneic and 24 autologous recipients. In the allogeneic recipient group, the majority of the patients were diagnosed with acute myelogenous leukemia (37%) and in the autologous group, non-Hodgkin's lymphoma (58 %). Sixty-five of 76 recipients (85.5%) had a total of 118 infectious complications. Out of the 52 allogeneic and 24 autologous recipients, 87 and 31 infectious complications occurred, respectively. Clinically defined infections were reported in 88 cases, microbiologically defined infections in 20 cases, and unexplained fever in 10 cases. Seventy-two infections occurred within the first 30 days following transplant, 17 cases between days 30 and 100, and 29 cases after the 100th day. Infection of the oral cavity occurred in 25.9% of the subjects, pneumonia in 24.1% and skin and soft tissue infection in 19.4%. Oral mucositis was the most common type of infection within the first 30 days following transplant, pneumonia between days 30 and 100, and skin and soft tissue infection after the 100th day. The causative organisms for bacteremia were gram- positive organisms in four of the cases and gram-negative organisms in six of the cases. Similarly, the causative organisms for pneumonia were cytomegalovirus in 5 cases, Pneumocystis carinii in 1 case, methicillin- resistant Staphylococcus aureus in 1 case, and M. tuberculosis in 2 cases. The most common cause of death was acute respiratory distress syndrome due to pneumonia (11 cases). CONCLUSION: Infection is a major complication in patients undergoing BMT. Infection occurred most commonly within the first 30 days following transplant, with oral mucositis and pneumonia being the most common types of infection. Antimicrobial prophylaxis with improved strategies should be utilized in order to prevent infection during post-BMT immunohematopoietic recovery.