The study was performed to explore the frequency of infections present at death and infection as the main cause of death (fatal infection - FI) in 845 consecutive patients (pts) treated for small cell lung cancer (SCLC) at the Institute of Tuberculosis and Chest Diseases in Warsaw, in the period 1980-1994. Diagnosis of infection was based on clinical signs and symptoms, the presence of new lesions on the chest X-ray, microbiological tests and/or autopsy examination. All cases of fungal infection, Pneumocystis carinii pneumonia (PCP) and tuberculosis were proved by autopsy and microscopic examination (including special staining). FI was diagnosed if no progression of cancer was noted and no other complications occurred. Infection was present at the time of death in 116 patients (13.7%) and FI was the cause of death in 39 of them (4.6%). Nine patients died from fungal infection, eight from bacterial infection, seven from PCP and two from tuberculosis. In 13 cases the aetiology of infection found at autopsy was not determined. All FI patients received chemotherapy and corticosteroids, 16 of them also had radiotherapy on the tumour and mediastinum. Thirty-two out of 35 patients had leucopenia. The risk of death from infection was greater in patients above 60 years of age. Patients in bad performance status died of infection significantly earlier than others (P<0.05).