The effects of vegetarian fasting were evaluated in 14 grossly obese patients who participated in a program comprising 5 weeks' fasting in a lactovegetarian health center. Before and after the fasting period the patients were hospitalized and put on a standardized weight-maintaining diet; at the health center they consumed vegetable juices containing less than 1 MJ and 3 g of protein per day. The weight reduction (mean +/- S.D.) was 13.4 +/- 5.0 kg (from 132.0 +/- 27.2 to 118.6 +/- 16.1 kg). Except for the first few days the patients had no severe hunger sensations. No severe adverse clinical effects were noted. The laboratory status--comprising serum or plasma levels of minerals, protein, and lipids; hematological data; and variables reflecting liver and thyroid function--revealed abnormal group mean values only for ferritin and the acute-phase reactants haptoglobin, C-reactive protein, and anti-chymotrypsin in the obese. The levels of potassium, retinol-binding protein, and haptoglobin decreased, and aminotransferase and lactate dehydrogenase activities and free fatty acid and glycerol concentrations increased as a result of the fasting. The most striking effect of the weight reduction was an increase in the HDL cholesterol levels. Fasting according to the described regimen thus seems to provide a safe method for treatment of obese patients.