To evaluate the clinical usefulness of four lung cancer-associated antigens ( LA-3, LSI-4, LSI-a, and LSI-c ), serum antigens in 107 patients with lung cancer and 35 patients with benign pulmonary diseases were measured quantitatively using a sandwich enzyme immunoassay. Pleural effusions from 20 lung cancer and 11 benign disease patients were assayed in the same manner. Mean levels of and positivity rates for each antigen in the sera of lung cancer patients were higher than in benigh diseases. Mean levels and positivity rates in stage III and IV lung cancer were significantly higher than those in stage I or II. Thus, diagnostic accuracy in lung cancer can be improved with combination assays for LA-3, LSI-4, and LSI-a. Each serum antigen level before and after chemotherapy changed according to the therapeutic effect. Indeed, the LA-3 antigen levels monitored in 3 lung cancer patients closely reflected the clinical course. Mean levels of and positivity rates for LA-3 and LSI-4 antigens in pleural effusions from lung cancer patients were significantly higher than for benign diseases. These results indicate that these 4 antigens detected in serum or pleural effusions can be clinically useful as tumor markers in lung cancer.