Serum concentrations of high density lipoproteins (HDL)-cholesterol, apoprotein A and apoprotein B has been studied in 13 patients with chronic renal failure treated with hemodialysis and in 12 patients with functioning renal graft. 5 patients on hemodialysis and 6 with a renal graft had hypertriglyceridemia. In the hemodialyzed patients, the increase in triglycerides was associated to a decreased concentration of serum apoprotein B. HDL-cholesterol was substantially reduced in the hemodialyzed patients without this being associated with a drop in serum apoprotein A concentration, thus suggesting that there are changes in the qualitative composition of the HDL. In renal graft recipients, the HDL-cholesterol and the apoprotein A concentration were increased. It is suggested that the risk of developing accelerated atheromatosis in patients carrying a renal graft cannot be related to changes in HDL. In patients undergoing hemodialysis, the development of atheromatosis would be related to decreased concentration and also qualitative changes in the HDL together with the increase in the VLDL and LDL.