Data from 6,569 middle-aged men in the Multiple Risk Factor Intervention Trial were analyzed to determine whether the weight change associated with smoking cessation resulted primarily from appetite or metabolic alterations. The appetite hypothesis attributes weight gain to an enhanced appetite and subsequent increase in caloric intake. The metabolic change hypothesis attributes weight gain to a metabolic alteration and subsequent decrease in basic caloric needs. Caloric intake and weight changes were tabulated for men who quit smoking and were compared to similar changes in men who continued smoking over 12 months. The difference between caloric intake changes in men quitting smoking versus men continuing smoking, controlled for weight change, was attributed to the metabolic change hypothesis. Men who quit smoking consumed 103 calories per day more (95% confidence interval = 29 to 177) than men who continued smoking with similar body weight changes. The increase in caloric intake attributed to smoking cessation was proportional to the number of cigarettes smoked prior to cessation.