Bacterial infections are a well-described complication of AIDS. However, relatively few reports have described infections due to Pseudomonas aeruginosa in adults who are infected with the human immunodeficiency virus (HIV). Seven cases of serious P. aeruginosa infection in HIV-infected patients occurred during 12 months in two hospitals in Houston, often in the absence of other host factors that are generally thought to predispose to this condition. One patient had no prior illness or antibody test results that were suggestive of HIV infection; for two other patients who were known to have antibody to HIV, an AIDS-defining diagnosis had never been made. Three patients had pneumonia (two with bacteremia and one with empyema), one had malignant otitis externa, and three had bacteremia that either resulted from or caused secondarily a soft-tissue focus of infection. Two patients died, and two others experienced one or more relapses after an initial course of treatment. Compromised host defense mechanisms, including loss of mucosal integrity, defects in humoral and cellular immunities, and qualitative or quantitative leukocyte abnormalities, may predispose HIV-infected patients to P. aeruginosa infections.