Patients with heterozygous familial hypercholesterolemia have a 50% deficiency of receptors for plasma low density lipoproteins (LDL) that induces a marked increase in plasma LDL levels. Two therapeutic measures that seem to increase the synthesis of LDL receptors are interruption of the enterohepatic circulation of bile acids with either bile-acid sequestrants or the ileal-exclusion operation, and competitive inhibition of 3-hydroxy-3-methylglutaryl coenzyme A reductase with mevinolin or compactin. To determine the effectiveness of this combination and the mechanisms of lowering LDL levels, we measured turnover rates of LDL apoprotein (apo-LDL) before and during treatment with mevinolin and colestipol in eight patients with heterozygous familial hypercholesterolemia. Drug therapy reduced LDL cholesterol levels by an average of 52%; this response was due to a 40% increase in fractional catabolic rate of apo-LDL and a 26% decrease in its production rate. A similar response was obtained in two patients who had previously had an ileal-exclusion operation for severe hypercholesterolemia and who were treated with mevinolin.