Heterozygous individuals with familial hypercholesterolemia possess about half of the normal numbers of functioning receptors on their cells. This is thought to be responsible for their hypercholesterolemia. In normals, dietary cholesterol increases LDL production and decreases LDL receptor-related LDL clearance, resulting in elevations in LDL cholesterol levels of approximately 30 mg/dL. To assess the effects of high fat and high cholesterol diets on the lipoproteins of individuals with diminished LDL receptors, three kinds of diets, including ones high in cholesterol, were fed to four patients with familial hypercholesterolemia, in the expectation that diet effects on apoB- or apoE-containing lipoproteins would be exaggerated. The basal diet consisted of 15% protein, 30% fat, 55% carbohydrate, 300 mg/d cholesterol, P/S ratio 0.4; the high fat diet was identical except that fat calories were 55% and carbohydrate 30%; the high fat-high cholesterol diet was identical with the high fat diet except approximately 750 or approximately 1,500 mg/d of cholesterol were added. Each diet was eaten for five weeks at home and for the sixth week at the general Clinical Research Center. Fasting (12-14 hours) plasmas were collected every two weeks for lipoprotein-lipid and apoprotein quantitation. At the end of each period, fasting and 4-hour postprandial samples were analyzed also by zonal ultracentrifugation and gel permeation chromatography.(ABSTRACT TRUNCATED AT 250 WORDS)