Variations in lipoprotein(a) [Lp(a)] levels were evaluated during the course of the nephrotic syndrome in 20 children (17 males, 3 females, aged 2-16 years), to evaluate the use of this parameter in the prognosis and monitoring of the disease. One patient was in relapse, 12 in remission, and 7 alternated between remission and relapse. Results were compared with those obtained in a control population of 100 age-matched children. Lp(a) was measured by a previously described immunonephelometric technique. Serum Lp(a) levels were increased in children with relapsing nephrotic syndrome compared with controls (median value of 419 mg/l vs. 86 mg/l). The median Lp(a) level in patients with nephrotic syndrome in remission was different from controls (270 mg/l under steroid therapy and 163 mg/l without steroid therapy), but remained within the reference range. Of the patients in relapse, 66% had Lp(a) levels above the generally accepted limit for cardiovascular risk of 300 mg/l, compared with 16% of controls, 44% of patients with nephrotic syndrome in remission under steroid therapy, and 18% of patients with nephrotic syndrome in remission without steroid therapy. In 2 patients, Lp(a) was measured repeatedly and was significantly higher during the acute phase of the disease (up to sixfold basal level). Changes in Lp(a) levels correlated with cholesterol levels, but the kinetics and the extent of variations were different. These data suggest that measurement of Lp(a) is useful for monitoring the nephrotic syndrome in children, particularly for detecting complications.