The plasma concentration of unesterified and esterified cholesterol within very low density (VLDL), low density (LDL) and high density (HDL) lipoproteins have been examined in relation to the metabolism and pool size of cholesterol in normal and hyperlipidaemic subjects. Cholesterol metabolism was assessed as faecal endogenous neutral and acidic steroid excretion, a 2-pool model of cholesterol turnover, and in vitro plasma cholesterol esterifying activity. VLDL total cholesterol (TC) concentration was positively correlated with cholesterol turnover, endogenous neutral steroid excretion, bile acid excretion and the absolute rate of plasma cholestrol esterification. The correlations with cholesterol turnover and neutral steroid excretion, but not that with bile acid excretion, remained significant when these were corrected for their relationships to body weight. LDL-TC was negatively correlated with the fractional rate of plasma cholesterol esterification and, in subjects with primary type IIa hyperlipoproteinaemia, also with the rate constant for cholesterol elimination from the rapidly exchanging cholesterol pool. No correlation was found between LDL-TC concentration and bile acid excretion. HDL-TC concentration was negatively correlated with both the rapidly and slowly exchanging pools of tissue cholesterol, after correction for their relationships to body weight and adiposity. In contrast, cholesterol pool sizes were not correlated with the concentration of VLDL or LDL-TC; nor was there any relationship to plasma cholesterol esterifying activity. No correlation was found between the relative proportions of unesterified cholesterol within any lipoprotein fraction and either the pool size or metabolism of cholesterol. These findings accord with previous reports of enhanced cholesterol metabolism in subjects with elevated VLDL concentrations and of impaired plasma LDL and cholesterol clearance in patients with primary type IIa hyperlipoproteinaemia. The demonstration that HDL-TC concentration is negatively correlated with body cholesterol pool size supports in vitro evidence for a role of HDL IN TISSUE CHOLESTEROL CLEARENCE.