Although hyperlipidemia is a well recognized complication of the nephrotic syndrome, the precise disturbances of lipoprotein metabolism which cause the elevated plasma lipid and lipoprotein concentrations have not been clearly defined in humans. This study examines the metabolism of apolipoprotein B-containing lipoproteins in patients with nephrotic-range proteinuria and in healthy controls. Two radioiodinated tracers of very low density lipoproteins (VLDL1, Sf60 to 400, and VLDL2, Sf20 to 60), were used to trace the metabolism of apolipoprotein B through the delipidation cascade from very low density lipoproteins (VLDL) to low density lipoproteins (LDL). The data from the apoB specific radioactivity curves and the pool sizes of apoB in four subfractions were analyzed by a multicompartmental modeling procedure using the SAAM 30 program. The main findings in the nephrotic group were: 1.) a consistent decrease in the fractional rate of apoB transfer from VLDL1----VLDL2 (median values-nephrotic 0.92 pools/day vs. controls 3.66, P less than 0.02) and from VLDL2----IDL (1.49 vs. 2.74, P less than 0.05); 2.) increased secretion of apoB into VLDL2 (14.5 mg/kg/day vs. 4.2, P less than 0.02); 3.) a trend towards decreased removal of IDL and LDL attributable to a defect in LDL receptor-mediated removal as previously shown (Metabolism 39:187-192, 1990). These findings suggest that catabolic defects of the apo B-containing lipoproteins are as important as increased hepatic synthesis in the pathogenesis of nephrotic hyperlipidemia in humans.