Two hundred and ninety-eight patients with limited (confined to chest and supraclavicular area, encompassable by a single radiation portal) small cell carcinoma of the lung were entered on Southwest Oncology Group Protocol 7628. Patients were treated with multi-agent chemotherapy and radiation therapy with or without BCG. Radiation therapy quality control analysis, including dosimetric reconstruction and port film review was introduced after the protocol was activated and was retrospectively applied. Patients who were considered major protocol variations had statistically worse survival (40 weeks versus 60 weeks; P = 0.002), a lesser improvement in response rate after induction chemotherapy (27 versus 48%; P = 0.05) and a higher chest failure rate (77 versus 55%; P = 0.047) than evaluable patients. Five patients relapsed in the brain, all associated with chest failure. Quality control is essential in cooperative group studies.