Many people use smoking as a weight control mechanism and are averse to quitting for fear of weight gain. These weight-concerned smokers tend to be women, are significantly less likely to stop smoking or to join smoking cessation programs, and will relapse more often than smokers who are not weight-concerned. Research suggests that a woman's motivation to quit smoking correlates positively with her confidence in her ability to control her weight after quitting. Likewise, success in smoking cessation has been associated with increased self-efficacy for weight control. This randomized controlled trial investigated the effects of a weight control program on eating and smoking behaviors in a group of female, weight-concerned smokers from July 2005 to June 2006. Two hundred sixteen subjects who wanted to lose weight but were not yet ready to quit smoking were recruited to participate in a 12-week, cognitive-behavioral weight control program consisting of 12 1-hour sessions. Subjects were randomly assigned to either the weight-control program (intervention group) or the control group. Differences between the intervention and control groups were evaluated using t tests for continuous variables, Wilcoxon rank-sum tests for ordinal variables and chi(2) tests for categorical variables. The intervention group had a 14% increase (P<0.001) in self-efficacy for weight control (Weight Efficacy Life-Style Questionnaire), which was associated with improved diet quality (Healthy Eating Index) (r=0.292, P<0.01), weight loss (r=0.582, P<0.001), increased self-efficacy for quitting smoking (Smoking Self-Efficacy Questionnaire) (r=0.291, P<0.014), a decrease in number of cigarettes smoked (r=0.331, P<0.005), and positive movement in stage of change toward smoking cessation (r=0.435, P<0.001). These findings suggest that for this group of weight-concerned smokers' success in changing eating behavior may trigger a positive change in smoking behavior.