Wilms tumor is the most common renal malignancy of childhood. Because of the improvement in prognosis and the increase in survival rates, long-term consequences of the treatment for Wilms tumor are of greater concern. We investigated late renal effects of the treatment on 50 survivors of nonsyndromic unilateral Wilms tumor. After the second year since the cessation of treatment, the glomerular filtration rate (GFR), urinary protein excretion, urinary beta 2 microglobulin levels, and blood pressure as well as the general health status were assessed. Results were analyzed for correlation with clinical variables, chemotherapy, and radiotherapy as possible risk factors. At a median follow-up time of 8.8 years (mean = 10.9; range, 2.3 to 35.4 y), none of the patients developed end-stage renal disease. Compensatory hypertrophy was observed in 68% of the cases. The median maximum bipolar length was significantly higher in patients diagnosed after the age of 36 months. Eleven (22%) and 2 (4%) of the 50 patients were hypertensive at the time of the diagnosis and the study, respectively. Similarly, median GFR values were significantly lower at the time of diagnosis, although at the time of the study, all patients had normal GFR values. With longer follow-up intervals, especially after 10 years, a significant decreasing trend in the GFR was observed (P = 0.002).