Fifty-nine evaluable patients with stage III bronchogenic carcinoma, participating in a randomized clinical trial evaluating the effect of adjuvant immunotherapy with levamisole or BCG in the treatment of clinically advanced lung cancer, were studied for their immunocompetence by in vitro and in vivo assays. Immunological tests consisted of measurements of natural killer (NK) cell and killer (K) cell cytotoxicity, skin testing reactivity to recall antigens, absolute lymphocyte count, and serum immunoglobulin (Ig) levels. Pretherapy K cell cytotoxic levels, skin test reactivity to trichophyton antigen, and increased IgA levels were predictive of the overall clinical course. Despite non-specific immunotherapy, progressive decline of NK and K cell cytotoxicity occurred during the course of the disease. These findings, however, were of limited clinical value. Initial performance status and disease extent significantly influenced time to progression and survival. Little further prognostic information was obtained from the immunological tests over those provided by clinical performance status and disease extent. No statistically significant differences were found in either time to progression or survival between controls and patients receiving either levamisole or BCG.