A protein-sparing modified fast (PSMF), which is a total fast modified by the intake of 1.2-1.4 gm. protein per kilogram ideal body weight (IBW), fluids ad libitum, and vitamin and mineral supplementation, allows effective control of carbohydrate metabolism and hunger. It reduces serum glucose and insulin concentrations in obese diabetic patients and increases free fatty acid and ketone body concentrations; ketonuria appears within 24-72 hours. When this fast was applied to seven obese adult-onset diabetics who were receiving 30-100 units of insulin per day, insulin could be discontinued after 0-19 days (mean, 6.5). In the three patients who had extensive nitrogen-balance studies, balance could be maintained chronically by 1.3 gm. protein per kilogram IBW, despite the gross caloric inadequacy of the diet. The PSMF was tolerated well in an outpatient setting after the initial insulin-withdrawal phase had occurred in the hospital. Significant improvements in blood pressure, lipid abnormalities, parameters of carbohydrate metabolism, and cardiorespiratory, symptoms were associated with weight loss and/or the PSMF. For diabetics with some endogenous insulin reserve, the PSMF offers significant advantages for weight reduction, including preservation of lean body mass (as reflected in nitrogen balance) and withdrawal of exogenous insulin.