Six hundred sixty-eight obese outpatients, 71 per cent (+/- 34) in excess of ideal weight, were enrolled in a multidisciplinary weight control program. The major components of the program included nutrition, education, behavior modification, and exercise. Rapid weight loss was accomplished using a very low calorie (less than 800 kcal) ketogenic diet. Patients adhered to the protein sparing modified fast (PSMF) for 17 +/- 12 weeks and averaged 9 +/- 17 weeks in a refeeding/maintenance program. Mean weight loss was 47 +/- 29 lb (21 +/- 13 kg) at the point of minimum weight and 41 +/- 29 lb (19 +/- 13 kg) at the end of the maintenance period. Systolic and diastolic blood pressure and serum triglycerides fell significantly in men and women. Success in weight loss was greatest in the heaviest patients, those who adhered the longest to the PSMF, and those who stayed the longest in the maintenance program.