We have analyzed the clinical significance of secondary infections associated with lung cancer patients. The incidence of secondary infections was 51.4% in 214 in-patients with lung cancer admitted to our institution in 1988 and 1989, and almost all of them had respiratory tract infections. The incidence was high in patients with cell types other than adenocarcinoma, and in those with hypoproteinemia, impaired cellular immunity and obstruction of the airway. The prognosis in patients with infection was much poorer than that in patients without infection. Major causative pathogens were Staphylococcus aureus including methicillin-resistant S. aureus (MRSA), Haemophilus influenzae, Klebsiella spp. and Pseudomonas aeruginosa. These pathogens except for H. influenzae were isolated at the terminal stage, in cases with airway obstruction and in post cancer-chemotherapeutic phase. The efficacy rate of 194 chemotherapeutic regimens against infection was 57.7%. Although the efficacy rate in 1988 and 1989 exceeded that in the 1970s, there was no significant difference in the efficacy rate between monotherapy (57.1%) and combined therapy (59.3%). The effectiveness was very poor for infections caused by P. aeruginosa and MRSA, or for cases with airway obstruction and marked impairment of pulmonary blood flow. The above results showed that a new combined therapy as well as the measures to improve the general condition of compromised hosts are required in the treatment of secondary infections in these patients.