To evaluate the efficacy of Krestin (PSK) as adjuvant treatment after radical radiation therapy (RT) for non-small cell lung cancer (NSCLC), treatment results of 225 patients with NSCLC treated with RT followed by adjuvant administration of PSK between 1976 and 1989 were analyzed. Of these patients, 170 (76%) had squamous cell carcinoma. In the patients with squamous cell carcinoma of the lung, PSK was given only when the tumor showed satisfactory shrinkage (complete or partial response) after completion of RT. The treatment outcomes were compared with those of the responders to RT not receiving PSK. The 5-year survival rates of patients with stages I-II and stage III disease were 39 and 26%, respectively, while the non-administered responder group's were 17 and 8%. These differences are statistically significant. An improvement in the treatment results with combined use of appropriate immuno-modulating drugs is anticipated in the future. When clinical trials of the efficacy of these drugs are conducted, the agents should be given to the patients with satisfactory tumor regression after RT, although they still take much time and cost.