Lipids of the blood serum were studied in 29 patients with untreated nephrotic syndrome (NS) and in 28 patients treated with corticosteroids or nonsteroid drugs. None of the patients had evidence of renal failure, either acute or chronic. The patients with untreated NS showed massive proteinuria, marked hypoproteinemia, considerable hypertriglyceridemia and hypercholesterolemia. Serum high-density lipoprotein cholesterol (HDL cholesterol) concentrations were lower in these patients than in the control group, including 35 normal subjects, and correlated with the total serum protein (r = 0.46, p < 0.05) and serum albumin (r = 0.46, p < 0.05). An inverse correlation was observed between HDL cholesterol and serum triglyceride levels (r = -0.58, p < 0.01). In the treated patients the laboratory indices of NS were less pronounced. HDL cholesterol levels were within normal limits in 14 patients with NS treated mostly with nonsteroid drugs, while in the patients receiving the corticosteroids (14 subjects) they were significantly higher than in the control group.