We determined the incidence of tuberculosis in lung cancer patients with known tuberculin status. All patients received chemotherapy with or without radiation therapy, corticosteroid therapy, or both, and none received isoniazid prophylaxis. Positive tuberculin reactivity was found in 89 of the 257 patients; among these 89, tuberculosis developed in one patient before and in one patient after chemotherapy. Among the other 168 patients, one case of tuberculosis developed after chemotherapy. For all lung cancer patients, the incidence of tuberculosis was higher than age-specific and race-specific rates in a control population. The risk of tuberculosis was judged to be significantly higher in patients with positive than with negative tuberculin reactivity; among tuberculin reactors, however, the risk of tuberculosis was estimated to be less than the potential risk of isoniazid hepatotoxicity reported in the literature for patients in a similar age group. The median survival for tuberculin-positive lung cancer patients was 9.6 months. Because of the limited survival in these patients, and because of the high risk of isoniazid hepatotoxicity for patients in this age group, we do not recommend isoniazid prophylaxis for tuberculin-positive patients receiving chemotherapy for lung cancer.