One hundred sixty-nine patients were entered into randomized controlled studies of intrapleural bacille Calmette Guérin (BCG) immunotherapy after surgical resection of lung cancer. Long-term follow-up of our initial series of BCG-treated patients with Stage I disease continues to indicate that this treatment was superior to that given to control patients. The recurrence rate in the control population was high, 62% at 3 years. The recurrence rate was 33% at 3 years in the BCG-treated group. A negative preoperative tuberculin test and squamous cell histologic type were favorable prognostic factors for BCG-treated patients. The survival of patients with more advanced disease was not improved by BCG immunotherapy.