To evaluate the long-term renal toxicity of cisplatin, 40 children who had been without treatment at least 18 months (range 18 months to 7 years) were observed. In all the children, glomerular filtration rate (GFR) was estimated from the plasma clearance of chromium 51-labeled ethylenediaminetetraacetic acid, both at the end of treatment and at a median follow-up of 2 years 6 months after treatment was stopped (range 18 months to 7 years). In 21 children, serum magnesium level was also measured at follow-up. Median age at diagnosis was 15 months (range 13 days to 13 years 8 months), and median cumulative doses of cisplatin was 500 mg/m2 (range 120 to 1860 mg/m2). In 22 of 24 children with an end-of-treatment GFR of less than 80 ml/min per 1.73 m2, the median improvement in GFR at follow-up was 22 ml/min per 1.73 m2 (range 2 to 56 ml/min per 1.73 m2). Hypomagnesemia was found in 6 of 21 children and was independent of GFR. No significant correlation was found between improvement in renal function and total cisplatin dose, age, gender, tumor type, or associated nephrotoxic medication. We conclude that most children have some recovery from cisplatin glomerular toxicity, especially if damage is not severe, but that hypomagnesemia may persist.