The concentrations of triglycerides and cholesterol have been determined in total serum and in the three major serum lipoprotein classes--very low (VLDL), low (LDL) and high (HDL) density lipoproteins-after an overnight fast in 39 patients with chronic uremia of more than 2 years' duration and with serum creatinine above 350 mumol/l. The values were compared with data from healthy male and female controls. The findings were similar for male and female uremics. Hypertriglyceridemia was common while serum cholesterol tended to be normal or subnormal. With the conventional typing system for hyperlipidemia, types II A, III and IV were present in 6,9 and 30%,respectively. The tryglyceride and cholesterol concentrations in VLDL were increased, while their normal relation for this lipoprotein class was maintained. In LDL the concentration of tryglycerides was increased, while that of cholesterol was low. The LDL composition, therefore, was changed to be more triglyceride-rich than normal. The changes in concentration and composition of LDL indicated that the levels of LDL 1 were raised and of LDL 2 decreased in chronic uremia. Increased levels of LDL tryglycerides occurred more frequently (40%) than increased levels of VLDL triglycerides (33%). The most striking and consistent lipoprotein abnormality was a low HDL cholesterol, which was not related to high VLDL levels. The HDL tryglycerides, on the other hand, tended to be somewhat high. The importance of the raised levels of the triglyceride-rich VLDL and LDL 1 and the decreased levels of HLD cholesterol for the rapid development of atherosclerotic vascular diseases which occur in chronic uremia is discussed. It is of interest in this context that both total cholesterol and LDL cholesterol were low. The possible mechanisms underlying the lipoprotein abnormalities in chronic uremia are discussed and it is suggested that they are complex.