Lipoprotein(a) [Lp(a)] is a plasma lipoprotein whose structure and composition closely resemble that of low-density lipoproteins, but contains an additional protein called apolipoprotein(a) [apo(a)]. Factors which modulate plasma Lp(a) concentrations are poorly understood. The influence of nephrotic syndrome on Lp(a) levels was investigated in 103 patients with nephrotic syndrome: 72 with primary kidney disease and 31 with diabetic nephropathy. Nephrotic patients had significantly higher Lp(a) levels (mean 63 +/- 7 mg/dl; median 42 mg/dl) compared with controls (mean 22 +/- 2 mg/dl; median 8 mg/dl). Fifty-seven percent of the patients and 22% of the controls had values greater than 30 mg/dl. Within all apo(a) isoform classes, higher concentrations of Lp(a) were seen in the nephrotic patients compared with controls. In 17 patients with primary kidney disease remission of the nephrotic syndrome was induced by immunosuppressive treatment and Lp(a) concentration dropped in parallel with the reduction of proteinuria (pretreatment mean, 98 +/- 9 mg/dl vs. remission mean, 25 +/- 5 mg/dl). In 9 patients where multiple measurements were done, multiple regression analysis showed a strong relation of Lp(a) with the amount of proteinuria (p < 0.01). We conclude that most patients with the nephrotic syndrome have Lp(a) concentrations which are substantially elevated compared with control subjects of the same apo(a) isoform. Because Lp(a) concentrations are substantially reduced when remission of the nephrotic syndrome is induced by immunosuppressive drugs, it is likely that nephrotic syndrome directly results in elevation of Lp(a). The high levels of Lp(a) in nephrotic syndrome could potentially cause glomerular injury as well as increase the risk of atherosclerosis and thrombotic events associated with this disorder.