Bacterial pneumonias are the most common pulmonary complication in HIV-infected patients. Up to now, H. influenzae and S. pneumoniae have been described as the most important germs. Within a period of 4 years we diagnosed 15 cases of pneumonia caused by P. aeruginosa. All patients were in HIV stage C3; 3F, 12M; median age 34 (24-54) years; median CD4 count 10 (0-130) microliters. Except for 3 nosocomial pneumonias, all others were community-acquired. Only 3 patients had neutropenia < 1000/microliter; 7 were intravenous drug abusers. Morphologically there were 6 cases of abscess pneumonia, in 3 of which pleural drainage was necessary because of pyopneumothorax. 4 patients showed bilateral infiltrates that could not be differentiated from those of P. carinii pneumonia. Our diagnosis was based on quantitative cultures of broncho-alveolar lavage fluid (9 cases, two of them with concurrent positive blood cultures/positive cultures of the pleural fluid), pleural puncture (one case), sputum in pneumonias responding only to antipseudomonas therapy (3 cases), and autopsy (2 cases). 8 patients died of pseudomonas pneumonia within 1-3 months despite therapy. 7 patients received pseudomonas-specific combination therapy, but all died after median 9 (4-15) months of the underlying illness. In 3 cases recurrent pseudomonas pneumonia could be documented bacteriologically. We conclude that in HIV-infected patients pneumonia caused by P. aeruginosa is a significant and severe pulmonary complication.