Although the majority of smokers are concerned about postcessation weight gain, few studies have investigated the prospective relationship between weight concern and smoking and weight outcomes, or a mechanism by which concern is related to these outcomes. We investigated the prospective role of smoking-specific weight concern in smoking cessation and weight gain among participants in a smoking-cessation clinic, and we hypothesized that domain-specific self-efficacy would be a mediator of these relationships. While weight concern did not prospectively predict smoking status, increased weight concern predicted weight gain at the end of treatment and at 3-month follow-up. Self-efficacy for preventing postcessation weight gain mediated this relationship; lower levels were related to a greater likelihood of weight gain. Weight gain was found to be associated with subsequent relapse among abstainers. Implications and treatment recommendations are discussed.