Although the acute renal toxicity of cisplatin has been well documented, long-term follow-up studies in cisplatin-treated children are scanty. We have evaluated the incidence and characteristics of both acute and chronic nephrotoxicity in 22 children (median age 8 years) treated with cisplatin as part of different chemotherapeutic protocols. All patients exhibited a significant and progressive decrease in plasma magnesium (Mg) values soon after cisplatin administration. Magnesiuria also increased immediately after therapy. Hypomagnesemia (plasma Mg < 1.4 mg/dl) occurred in 10 patients and it was dose-dependent. Minimal and mean cumulated doses inducing hypomagnesemia were 300 and 500 mg/m2, respectively. In 18 children we followed renal function prospectively for a mean time of 2.3 years after arrest of cisplatin therapy. Chronic hypomagnesemia and moderate elevation of plasma creatinine were observed in 6 children, hypocalciuria in 5 children, and hypokalemia in 1 child. Presence of hypomagnesemia was unrelated to the total dose received or the time elapsed since cisplatin therapy. Renal function studies, performed in the 6 children with chronic hypomagnesemia, revealed different degrees of impairment in Mg reabsorption. The functional characteristics of chronic cisplatin nephrotoxicity found in the present series-contrary to prior reports-are not comparable to those present in the inherited Gitelman's syndrome.