Pseudomonas cepacia is a gram negative rod, having no fermentative activity on glucose. This organism was detected in the sputum, throat swab, or throat washing of 22 inpatients treated between January, 1990, and December, 1990, at the First Department of Internal Medicine, Kagawa Medical School. The primary diseases for which these 22 patients were hospitalized were leukemia in 12, malignant lymphoma in 5, lung cancer in 2, myelodysplastic syndrome in 1, and embryonal cell carcinoma in 1. Twelve of the 22 patients had episodes of pneumonia which complied clinically with the diagnostic criteria provided to facilitate the National Nosocomial Infection Study. The complication of pneumonia occurred in 7 patients with leukemia, 2 with malignant lymphoma, 2 with lung cancer, and 1 with myelodysplastic syndrome. In 10 of these 12 patients, the organism was detected before the onset of pneumonia. All 22 patients in whom the organism was demonstrated had received antibiotics. The antibiotics which was most frequently used to treat these patients 1 month before detection of Pseudomonas cepacia were amikacin and ceftizoxime, which were used in 13 patients. Of the antibiotics in which the susceptibility to Pseudomonas cepacia was, evaluated, minocycline was effective in 100% (21/21), ceftazidime in 50% (11/22), and ofloxacin in 27.3% (6/22). Physicians should be especially aware of the possibility of colonization and nosocomial respiratory infection by Pseudomonas cepacia in patients with severe underlying diseases.