Eighteen patients with seasonal affective disorder (SAD) participated in a double-blind, placebo-controlled crossover study in 1986-1987. Each received, in random order, d-fenfluramine (15 mg p.o. twice daily)-a serotonin-releasing drug previously shown to suppress carbohydrate craving-or a placebo; these were given for 4 weeks separated by a 2-week washout period. Symptoms were assessed by means of clinical interviews and the Hamilton Rating Scale for Depression (HAM-D) with a special SAD addendum (AAD). Patients were also weighed. Depression scores (mean +/- SE) were identical before treatment with drug (20.9 +/- 1.3, HAM-D; 13.3 +/- 0.8, AAD) or placebo (21.4 +/- 1.2, HAM-D: 13.2 +/- 0.6, AAD). During placebo treatment, mean HAM-D scores declined by 22% (p less than .02) and AAD scores by 9% (p greater than .2). During d-fenfluramine treatment, HAM-D scores fell by 71% (p less than .001) and AAD scores by 73% (p less than .001). Thirteen (72%) of the patients demonstrated complete reversal of their abnormal test scores while taking d-fenfluramine. The group as a whole lost weight (mean = 1.2 kg) while receiving d-fenfluramine (p less than .033) but not when taking placebo. A second study, conducted in 1987-1988 with nine subjects who had previously responded to d-fenfluramine, showed that the drug remains effective for the full 3-month annual period of symptoms. These results indicate that d-fenfluramine may be useful in treating SAD and suggest that serotonin is involved in both SAD's affective and appetitive symptoms.