Lipids in erythrocytes and plasma of children and adults with terminal renal insufficiency were determined and compared with those of normal controls. The erythrocyte phospholipids in uremic patients were altered in phosphatidylethanolamine (absolutely and relatively elevated) and in lecithin (relatively diminished). Sphingomyelin, phosphatidylserine, total erythrocyte phospholipid content and cholesterol were within the normal range. The detailed analysis of the distribution of plasma phospholipids showed an increase in all phospholipids except phosphatidylserine. The unesterified plasma cholesterol was also found to be elevated. The fatty acid distribution in the individual erythrocyte phospholipids showed the following variations in comparison with controls: one main fatty acid was significantly decreased in sphingomyelin and in lecithin and slightly decreased in phosphatidylethanolamine. It could be shown that the lipid alterations in uremic erythrocytes were quite different from those in plasma. There were no typical signs for an increased lipid peroxidation in the erythrocytes of patients with renal insufficiency.