The major lipid disturbance in children with congenital nephrosis of the Finnish type (CNF) is hypertriglyceridaemia. To determine whether or not hypertriglyceridaemia is caused by defective triglyceride catabolism, we measured lipoprotein lipase (LPL) activities and masses at various stages of the disease. At age 3 months in CNF both LPL activity and mass were decreased, but a close positive correlation between these parameters similar to that in controls was observed. At age 9 months both LPL activity and mass were even lower. At that time a significant positive correlation (r = 0.72, P < 0.05) between LPL activities and albumin concentrations and significant negative correlations between plasma free fatty acid (FFA) concentrations and LPL activities (r = -0.72, P < 0.05) and between plasma FFA concentrations and serum albumin concentrations (r = -0.73, P < 0.05) were observed, suggesting that low albumin concentrations result in increase of FFA levels, which could interfere with a normal LPL function at the endothelial surface. On dialysis after nephrectomy, LPL activities and masses increased. At age 3 and 9 months apoprotein C-II (apo C-II) and apoprotein C-III (apo C-III) levels were not decreased although apoproteins were being lost into the urine. On dialysis the mean ratio of apo C-II/C-III was significantly lower than the mean in controls (P < 0.001). We conclude that impaired function of LPL seems to be the major cause of hypertriglyceridaemia and disintegrity of the VLDL-IDL-LDL delipidation cascade in children with CNF.