Cholesterol catabolism was measured in three obese hyperlipidemic human subjects before and during fasting by determining fecal end products of cholesterol metabolism, bile acids and neutral steroids derived from cholesterol. Serum cholesterol level was reduced by fasting in one subject and remained unchanged in two others. Bile acid excretion decreased in two, and neutral steroid excretion in all three subjects so that the sterol balance was reduced during weight reduction on an average by 40%. The slope of the serum cholesterol specific activity-time curve appeared to be decreased during caloric restriction, suggesting that the reduced catabolism of cholesterol was associated with an expanded extravascular pool size of exchangeable cholesterol or, more probably, with mobilization of cholesterol from lost tissues and reduced cholesterol synthesis. During hypocaloric diet the latter apparently potentiates the effect of drugs lowering serum cholesterol level.