In a multicenter study by 243 practicing physicians in Austria 819 severely obese subjects of both sexes without overt disease were encouraged to keep a calorie-restricted diet to reduce weight. After a run-in period of more than two weeks of dieting patients started taking 15 mg dexfenfluramine (Isomeride) twice daily for three month. While their weight was fairly stable during the run-in period progressive weight loss occurred during taking dexfenfluramine due to obvious changes in eating habits and appetite allowing to keep the reducing diet more strictly. Females lost 7.7 +/- 3.9 kg while obese men lost 9.32 +/- 4.6 kg. Laboratory tests obtained before starting dexfenfluramine and after 3 months at termination of medication showed blood glucose, cholesterol, LDL and triglycerides to decrease while HDL-cholesterol increased moderately. Dexfenfluramine was well tolerated by the majority of patients. Side effects such as fatigue, sedation, flatulence or diarrhea occurred in only 7.9% of the probands initially and dropped to 2.1% during the third month of the medication. It is concluded that Dexfenfluramine modifies eating habits and appetite thus making weight reducing diets easier acceptable and resulting in weight loss. It is suggested that Dexfenfluramine has a role in treatment regimes for morbid and refractory obesity.