Six E2E2 homozygous familial dysbetalipoproteinemic (FD) patients were treated with gemfibrozil (2 x 600 mg/d) for a period of 4 weeks. For all subjects, normalization of serum cholesterol concentrations with treatment did not result in a significant change in the cholesterol/triglyceride ratio of the d < 1.019 lipoprotein fraction. In addition, the binding efficiency of this lipoprotein fraction to the low-density lipoprotein (LDL) receptor on HepG2 cells did not change consistently with treatment. We conclude that normalization of serum cholesterol concentrations in FD patients by treatment with gemfibrozil is the result of an effect of gemfibrozil on the synthesis of d < 1.019 lipoproteins rather than an effect on the receptor-mediated clearance of these particles.