In a single-blind trial, 15 obese patients, selected because of their poor response to a weight reducing diet, were treated for 1 month with placebo plus a 1600 calorie diet and then for a further 2 months with a single daily dose of 120 mg fenfluramine plus diet. After the placebo period, the mean weight loss was only 0.89 kg. After the 2-month period on fenfluramine, there was a highly significant (p less than 0.001) additional mean weight loss of 10.18 kg. Biological investigations of the metabolic effects of fenfluramine showed that compared with the placebo period there was an improved glucose tolerance and insulin response after active treatment as well as statistically significant decreases in serum cholesterol (p less than 0.001) and triglycerides (p less than 0.01). During active treatment none of the patients complained of feeling hungry and only a few transient side-effects were reported at the start.