Pharmacologic measures which increase serotonergic activity in the brain decrease food consumption and lead to decreased weight in animals. Fluoxetine, an inhibitor of serotonin reuptake, decreases food intake in animals and is associated with weight loss in depressed and otherwise healthy obese patients. To determine the most effective daily fixed dose which causes weight loss in nondepressed obese patients, fluoxetine (10, 20, 40 or 60 mg) or placebo was administered once daily for 8 weeks to 655 patients consisting primarily of women (mean age 40 years, mean weight 95 kg). Diet and activity were not controlled. The placebo-treated patients lost 0.6 +/- 2.3 kg. With the 60-mg fluoxetine dose, patients lost an average of 4.0 +/- 3.9 kg (P less than 0.001), with intermediate responses at the lower doses. Weight loss was proportional to the initial body mass index (weight/height squared). There were no statistically significant differences between any fluoxetine treatment group and placebo for discontinuations from the study. There were statistically significant dose-dependent increases in reports of asthenia, somnolence and sweating. Thus, fluoxetine 60 mg daily appears to be potentially effective for use in weight reduction.