To describe why medical patients quit smoking and the methods they use.Cross-sectional and prospective cohort design. Patient smokers were enrolled in a study of physician counseling about smoking. One year later, 2,581 of the patients were asked about quit attempts and methods used. Of those, 245 former smokers whose quitting had been biologically validated were interviewed about why and how they had quit.Offices of internists and family practitioners in private practice and a health maintenance organization.Consecutive sample of ambulatory patients who smoked.Baseline questionnaires included demographic data, smoking history, and symptoms and diagnoses related to smoking. After one year, subjects were interviewed about smoking status and methods used in attempting to quit. Cessation was confirmed by biochemical testing. Those who had quit were asked about reasons for quitting. Seventy-seven percent of successful quitters gave health-related reasons for quitting and the quitters ranked "harmful to health" as the most important reason for quitting. In a multivariate analysis, those who had a college education, who had social pressures to quit, and who had greater confidence in being able to quit were more likely to have quit smoking one year later, while those who smoked their first cigarette within 15 minutes of awakening and who had more diagnoses related to smoking were less likely to have quit smoking one year later. Participation in a treatment program and having been counseled by a physician or nurse practitioner were positively related to successful quitting, while use of filters or mouthpieces was negatively related.Concerns about health are the most common reason patients give for quitting, and addiction is the most important barrier to quitting. Education, social pressure, provider advice, and formal programs, but not over-the-counter devices, appear to increase the chances that smokers will quit.