A prospective randomized trial was performed to evaluate the role of adjuvant local immunostimulation with bacillus Calmette-Guérin (BCG) in resected Stage I and Stage II non-small cell bronchogenic carcinoma. The patients were stratified according to extent of resection and surgical stage, and randomized to treatment or placebo. Adjuvant treatment consisted of a single dose of BCG (Tice) injected into the pleural space between days 6 and 12 postoperatively. Isoniazid was given by mouth for 12 weeks. In the control group saline was injected intrapleurally. Of 441 patients included in the study, 407 were evaluable, 198 of them in the BCG group and 209 in the control group. The average follow-up was 4.7 years. A high rate of complications was noted in the BCG group; after pneumonectomy 22% of these patients developed pleural empyema necessitating further surgical procedures, in comparison with 3% in the placebo group. There was no significant difference between the two randomized groups with respect to survival. There was, however, a significant decrease in the disease-free interval in patients who received BCG (P = 0.044). This detrimental effect with BCG was especially pronounced in pneumonectomized patients (P = 0.028). There was no significant difference between treatment and placebo in patients with lobectomies. Because of no proved benefit of regional administration of BCG (and even of detrimental effects after pneumonectomy) and a high rate of severe complications, the authors advise against the use of BCG intrapleurally as local adjuvant immunostimulation.