The aim of this study was to investigate the effects of valproate and carbamazepine, on renal glomerular and tubular functions. The patient group comprised 54 children with new-onset epilepsy treated with valproate (n = 30) and carbamazepine (n = 24). Twenty-six healthy children were in the control group. The serum creatinine and cystatin C levels and urinary excretion of N-acetyl--d-glucosaminidase (NAG) levels were measured and the glomerular filtration rate (GFR) was estimated. Serum creatinine and cystatin C concentrations were not different between patients and controls. The glomerular filtration rate of the patient groups were higher than those of the control group. Thus, both drugs probably lead to glomerular hyperfiltration and toxicity for glomerular functions. However, urinary N-acetyl--d-glucosaminidase/creatinine levels were significantly higher in patients receiving only valproate (6.1 +/- 5). The difference between carbamazepine and control groups was not significant for urinary N-acetyl--d-glucosaminidase/creatinine levels. Our data suggest that valproate has adverse effects on renal tubular functions.