Erythrocytes from patients with various disorders of lipoprotein metabolism have been found to have abnormal morphology. We report morphologic abnormalities of erythrocytes from two patients with homozygous familial hypercholesterolemia (HFH), in which knisocytes, stomatocytes and crenated cells were observed. The membrane lipid and phospholipid fatty acid composition of HFH erythrocytes was not significantly different from controls. HFH erythrocytes incubated in HFH patient plasma and a lipoprotein-rich fraction of HFH plasma appeared morphologically similar to erythrocytes from HFH patients. These studies support the concept that serum lipids exert an important role in the regulation of erythrocyte morphology in the normal state, as well as in patients with disorders of lipoprotein metabolism.