The study was designed to investigate the hyperlipidaemia associated with the steroid responsive nephrotic syndrome in children and in particular to examine the mechanism for the delayed clearance of the circulating triglyceride-rich lipoproteins. The possibility that plasma from patients with steroid responsive nephrotic syndrome may contain an inhibitor of lipoprotein lipase activity was studied by examining the effect of the addition of plasma from patients, on normal postheparin lipoprotein lipase activity. Plasma from children with steroid responsive nephrotic syndrome significantly inhibited lipoprotein lipase activity (p less than 0.001), whereas that from patients with familial hypercholesterolaemia and normal children had no significant effect. The inhibition of lipoprotein lipase activity by plasma from patients with steroid responsive nephrotic syndrome correlated significantly with their increased plasma cholesterol and reduced plasma albumin concentrations (p less than 0.001 and less than 0.02, respectively), but there was no significant correlation with plasma triglyceride concentrations. Thus, the degree of inhibition probably reflected the severity of the condition at the time of study. Neither the cholesterol, albumin nor triglyceride concentrations appeared to directly influence the lipoprotein lipase activity of postheparin plasma.