From May 1990 to August 1991, 36 patients admitted to the Department of Internal Medicine in a medical school hospital with hematological malignancies or solid tumors, developed respiratory tract colonization with Pseudomonas cepacia. Sixteen (44.4%) of these patients developed pneumonia, and four (11.1%) died of respiratory failure due to P. cepacia pneumonia. Extensive survey of the hospital environment as well as equipment showed that nebulizer devices used by the patients for inhalation were contaminated with P. cepacia. Phenotypic characteristics, (production of hemolysin and extracellular enzymes [lipase, lecithinase and protease]), the Analytical Profile Index 20 NE pattern, and the pattern of DNA fingerprinting by pulse-field gel electrophoresis in clinically isolated strains and strains derived from nebulizer devices were compared. The strains of P. cepacia obtained from patients in the Department of Internal Medicine were indistinguishable from each other and also from those isolated from nebulizer devices, but were different from those isolated from patients in other departments at the same time. These results demonstrated that the outbreak of P. cepacia respiratory colonization in immunocompromised patients was a nosocomial acquisition, and probably occurred by transmission through contaminated nebulizer devices.