We investigated lipoprotein profiles in 24 children with normal renal function at different stages of the idiopathic nephrotic syndrome (NS). Four groups of patients were studied: (I) steriod-resistant NS with persistent proteinuria; (II) untreated steroid-sensitive NS during a relapse; (III) steroid-sensitive NS in remission induced by steroid-treatment; (IV) steroid-sensitive NS in long-term remission without therapy. Triglycerides (TG), cholesterol (CHOL), and phospholipids (PLP) were measured in plasma as well as in the lipoprotein fractions of very low (VLDL), intermediate (IDL), low (LDL) and high density (HDL). Apoproteins (Apo) AI, AII, B and C-apoproteins were measured in patients of groups I and IV. Results were compared to those obtained in 24 healthy control subjects. All patients with active NS (groups I-III) had significantly elevated CHOL levels. TG and CHOL in the VLDL, IDL, LDL, and CHOL in HDL2, but not HDL3 were inversely correlated with the serum albumin level. Patients with active NS had increased concentrations of TG and CHOL in lipoprotein fractions of lower density. Total and fractionated HDL-CHOL was not significantly different from control levels in any group. Patients in group I had significantly reduced Apo AI levels, whereas an increase of Apo AI and Apo AII in HDL3 and of most C-apoproteins in both HDL fractions was observed in patients of group IV. While changes in HDL apoprotein composition during long-term remission are of yet unknown clinical significance, our data indicate an increased risk of atherosclerosis only in those paediatric patients with persistent steroid-resistant NS.