Exogenous estrogens (ethinyl estradiol, 1 microgram/kg body weight per day), which stimulate triglyceride production in normal women and those with endogenous hypertriglyceridemia, were found to exert a paradoxical, hypolipidemic effect in six subjects (five women, one man) with type III hyperlipoproteinemia on diets both of normal and of fat-free, high-carbohydrate composition. Moreover, very low-density (VLD) lipoprotein lipid and apolipoprotein composition and electrophoretic mobility became normal during estrogen administration in these subjects. Levels of normal VLD lipoproteins remained mildly to moderately elevated in a type IV lipoprotein pattern. Estrogen withdrawal promptly restored the type III pattern with its abnormal enrichment of VLD lipoproteins with apolipoprotein E (the arginine-rich peptide). These findings suggest that estrogens facilitate the assimilation of chylomicron and VLD lipoprotein remnants, a defect that appears likely to represent the metabolic abnormality underlying type III hyperlipoproteinemia.