There is limited research of smoking cessation following diagnosis of lung cancer. This prospective study assessed cigarette smoking behavior among 226 patients (142 males, 84 females) prior to, at the time of, and after the diagnosis of unresectable stage IIIA/IIIB non-small cell lung cancer and entry into a phase III trial examining combined thoracic radiation therapy and chemotherapy. Their mean ± S.D. Aage was 62.7 ± 9.4 years and 95.6% were Caucasian. Of 215 patients with a history of cigarette smoking, 69% (148/215) stopped smoking prior to entry in the trial, 9% (20/215) stopped smoking at some point during the course of the trial, 11% (24/215) continued smoking throughout the trial, 7% (16/215) were smoking at baseline but did not report subsequent smoking status, and smoking status at study entry was missing for the remaining patients. The majority of lung cancer patients were able to stop making. A notable subset of patients continued smoking despite diagnosis of lung cancer, enrollment in a clinical trial, treatment-related toxicity, and encouragement from clinicians to stop smoking. Smoking cessation interventions are needed for lung cancer patients who continue to smoke. Copyright © 2002 John Wiley & Sons. Ltd.