Using 24 lung cancer patients (20 males and 4 females) who had received radiotherapy, a multivariate analysis has been performed in relation to 19 factors suspected of exerting an effect on natural killer (NK) cell activity. As influencing factors, two factors of the disease, the stage (p less than 0.05) and the leukocyte count (p less than 0.08), were calculated to be significant. In the univariate analysis, the NK activity was found to be significantly low (p less than 0.05) when the leukocyte count was 7,000 or higher. In addition, patients who had died during the course of radiotherapy showed an NK activity which was significantly lower (p less than 0.05) than the activity manifested in both the disease-free patients and the relapsed progressive disease patients. Moreover, the patients with a PS of 40% or less had a significantly lower NK activity than the patients with a PS of 50% or more, though no correlation was found between the NK activity and the disease stage. In the use of this radiotherapy, the NK activity was found to be more strongly influenced by the size of the irradiated field than by the irradiated dose.