We studied the concentrations of total cholesterol, HDL-cholesterol and triglyceride in the plasma in renal (n = 48) patients. In undialyzed uraemic patients (mean serum creatinine value 439 mumol/l) the plasma cholesterol level was somewhat higher (6.47 mmol/l) than in the control (n = 27) group (5.67 mmol/l). HCL-cholesterol was lower (1.02 mmol/l vs 1.49 mmol/l) and the triglycerides higher (2.07 mmol/l vs 1.07 mmol/l) than in the control group. In dialyzed patients the cholesterol level (5.88 mmol/l) was lower but the triglyceride level higher (2.58 mmol/l) than in undialyzed patients. Renal transplantation corrected the lipid variables to a certain degree (plasma cholesterol 5.55 mmol/l, plasma HDL-cholesterol 1.27 mmol/1 plasma triglycerides 1.15 mmol/l) but HDL-cholesterol remained still somewhat lower than in the control group. No "critical level" of renal function at which the changes of lipoprotein metabolism arise, could be demonstrated, and it seems that e.g. HDL-cholesterol is low already when the diagnosis of renal insufficiency is made.