We examined retrospectively the pattern of respiratory infection in 579 patients with lung cancer admitted to Nagasaki University Hospital during the past 15 years. A total of 139 patients (24.0%) developed respiratory infection. The rates of pulmonary infection associated with large (36.2%) and small cell carcinomas (33.6%) were significantly higher than those with squamous cell carcinoma (26.0%) and adenocarcinoma (17.3%). Advanced stages of lung cancer were associated with higher complication rates (stage I: 6.3%, stage II: 15.9%, stage III: 27.9%, and stage IV: 33.8%). Deceased patients showed a significantly higher rate of pulmonary infection than alive patients during the period of investigation. Isolated organisms in excess of 10(7) cfu/ml in sputum or 10(4) cfu/ml in bronchial aspirate were mainly gram-negative bacteria (68.8%), such as Haemophilus influenzae, Klebsiella pneumoniae, Enterobacter cloacae, Acinetobacter sp. and Pseudomonas aeruginosa. The number of patients infected with gram-positive bacteria increased markedly after 1982. Our results suggest that a successful control of pulmonary infection associated with lung cancer is important in improving the prognosis of lung malignancy.