An attempt was made to interpret the clinical significance of secondary infections associated with lung cancer. The incidence of secondary infections was 51.4% in 214 in-patients with lung cancer in our institution in 1988 and 1989, and almost all of them had respiratory infections caused by commonly encountered bacteria. The incidence of infection was high in lung cancer of cell types other than adenocarcinoma, and in those with hypoalbuminemia, impaired cellular immunity and obstruction of the airway. The prognosis in patients with infection was much poorer than that in patients without infection. Major pathogens responsible for infection were Staphylococcus aureus including methicillin-resistant S. aureus (MRSA), Haemophilus influenzae, Klebsiella spp. and Pseudomonas aeruginosa. These pathogens, except for H. influenzae, were isolated in the terminal stage in cases with airway obstruction and post cancer chemotherapy. The efficacy rate of 194 therapeutic regimens against infection was 57.7%. It was thus found that the efficacy rate in 1988 and 1989 exceeded that in the 1970s. The effectiveness was very poor for infections caused by S. aureus and P. aeruginosa, and for cases with airway obstruction and marked impairment of pulmonary blood flow. The efficacy rate of single-drug regimens was 57.1% (80/140) and that of combined regimens was 59.3% (32/54). The above results indicate that a new combined therapy which includes a beta-lactam antibiotic as well as measures to improve the general health of compromised hosts are required in the treatment of secondary infections in these patients.