Twenty-five patients sustained an average weight loss of 21.7 pounds on a diet in which 73 per cent of total calories consisted of animal fat. Their "new" weight was maintained on this high-fat diet and subsequently on a low-fat diet. Standard Sf0-12 lipoproteins were highly influenced by dietary fat intake independent of weight change. In contrast the Sf20-400 fraction was lowered by negative caloric balance and weight reduction despite the ingestion of large amounts of animal fat. These changes were statistically highly significant. Divergent responses of different classes of beta-lipoproteins may be missed if only total beta-lipoprotein measurements are made.