In 36 patients suffering from AIDS (age: 21-62 years) the incidence, the efficiency of diagnostic procedures and the clinical course of bacterial pneumonias were analysed. 19 out of 20 episodes with bacterial pathogens were found in 18 patients via fibre-bronchoscopic sampling. The most frequently observe bacteria were S. pneumonia (n = 8), M. tuberculosis (n = 3) and atypical mycobacteria (n = 6). At least 2 pathogens of the pneumonia were observed in 9 episodes. An appropriate treatment with antibiotics on the basis of a proper microbiologic diagnosis was successful in 65% of the cases. Because of the unreliability of history, clinical findings, chest X-ray, and laboratory features, successful management of bacterial pneumonias of AIDS patients should be based on the knowledge of the causative agent which can be obtained by the efficient and safe procedure of bronchoscopic sampling.