The effects of fasting or a 600-cal diet on biliary lipid composition were studied in 20 persons with overweight. With complete fasting for 20 days and a mean weight loss of 650 g/day, biliary cholesterol fell significantly from 10.1 to 6.9 mol% and the 'lithogenic index' from 1.0 to 0.7. Following fasting, the index, in 10 of the 11 patients, was in the normal range below 1.0. With a 600-cal diet, in agreement with findings by other authors, there was no significant change of individual bile lipids or of the lithogenic index. While it has been shown that lithogenicity increases up to 16 h of fasting, it obviously significantly decreases with continued fasting. Fasting, therefore, in addition to manipulation of dietary cholesterol and, possibly, fiber consumption, appears to be an effective method for rapid lowering of biliary cholesterol saturation.