This paper presents results of a prospective study which examined factors influencing self-initiated smoking behavior change in a cohort of smokers followed over 17 months. Employees of a cancer hospital and research center were surveyed about smoking habits, health status, attitudes about smoking and social-environmental factors. Seventeen months later, employees identified as smokers were resurveyed about their current smoking status, recent attempts at cessation and use of cessation aids in these attempts. Overall, 47% of subjects had not tried to stop smoking (non-stoppers, NS), 38% had attempted to quit but returned to smoking (recidivists, R) and 15% had successfully quit (SQ). Predictors of attempting cessation (R and SQ versus NS) included smoking fewer cigarettes daily, starting smoking at a later age, previous attempts at quitting, lower nicotine dependence, greater pressure to stop smoking and an expectation to quit in the near future. Amount smoked daily was the strongest predictor of successful quitting (SQ versus R). Findings from this study suggest intention to stop smoking is the most important factor differentiating smokers who attempt cessation from those who do not. Strength of smoking habit appears to be the strongest factor associated with successful quitting.