The causes of primary moderate hypercholesterolemia are not understood, but some patients have reduced fractional clearance rates (FCRs) for low density lipoproteins (LDL). This could be due to either decreased activity of LDL receptors or to a defect in structure (or composition) of LDL that reduces its affinity for receptors. To distinguish between these causes, simultaneous turnover rates of autologous and normal homologous LDL were determined in 15 patients with primary moderate hypercholesterolemia. In 10, turnover rates of both types of LDL were indistinguishable, which indicated that autologous LDL was cleared as efficiently as normal homologous LDL. In five others, FCRs for autologous LDL were significantly lower than for homologous LDL. Two of the latter five were treated with mevinolin, and although FCRs for both types of LDL rose during treatment, differences in FCRs between the two types of LDL persisted. In these five patients, autologous LDL appeared to be a poor ligand for LDL receptors.