Between January 1988 and December 1992, 68 patients admitted to our Department of Internal Medicine with haematological malignancies or solid tumours showed colonization of the respiratory tract with Stenotrophomonas maltophilia. To characterize the significance of respiratory tract colonization by S. maltophilia, we retrospectively reviewed the medical records of the 68 patients colonized with this organism. Twenty-nine of these 68 patients developed pneumonia, with S. maltophilia being implicated in 10 cases. The majority of these 10 patients showed lobular infiltration on chest X-ray. Pleural effusion was observed in two (20%) of the 10 patients. All 68 strains of S. maltophilia were resistant to imipenem. Latamoxef was effective against 98·5% of strains, while minocycline was effective against 100% of strains. This report describes the clinical features of nosocomial S. maltophilia pneumonia in immunocompromised patients.