Fifty-five men and 58 women participating in a behavioral weight loss program were studied to determine sex differences in weight history, eating patterns, efficacy expectations, self-esteem, weight loss and maintenance. Weight history, eating patterns, and psychological variables were further analyzed to identify factors associated with weight change. Women reported more previous dieting experience and a higher degree of overweight at age 25 than men. Women also reported more eating in response to mood and lower self-efficacy prior to participation in this program, while men reported more eating in social situations. Women's self-efficacy rose during treatment, but men's did not. Short-term weight loss was not related to sex, but women were more successful in maintaining weight loss. Degree of overweight at age 25 and prior program participation were inversely related to weight loss success. Self-efficacy in social situations was positively associated in men with short-term weight loss. For men, eating frequency in various moods at post-treatment negatively predicted weight maintenance, and self-efficacy in controlling mood-related eating was positively associated with weight maintenance. We conclude that men and women differ importantly along several dimensions with regard to weight history and response to treatment. These differences may have implications for treatment of overweight.