Directly measured food intake in 31 overweight female smokers to test whether (a) calorie and carbohydrate intakes increase after smoking cessation and (b) double-blind d-fenfluramine (30 mg), a serotonin-releasing drug, suppresses weight gain, overeating, and dysphoric mood associated with stopping smoking. Placebo-treated patients grew dysphoric after smoking withdrawal and ate 300 kcal/day more from 2 to 28 days after, showing a 3.5-lb weight gain. Fat and protein intakes did not change, but carbohydrate intake increased (30% to 40%). D-fenfluramine prevented postcessation dysphoria. Although drug-treated patients ate more carbohydrate snacks just after quitting, they returned to baseline by 4 weeks, showing a 1.8-lb weight loss. Agents that enhance brain serotonin-mediated neurotransmission may help prevent weight gain, overeating, and dysphoric mood after smoking withdrawal.