BACKGROUND The correlation between the urinary 6beta-hydroxycortisol/17-hydroxycorticosteroids (6beta-OHF/17-OHCS) ratio and the metabolic capacity of the most abundant form of hepatic cytochrome P450 (CYP3A4) after induction remains unclear. METHODS Concentrations of 6beta-OHF and 17-OHCS in spot urine specimens obtained from 61 epileptic children receiving continuous carbamazepine therapy were measured by high-performance liquid chromatography. The relationship between the urinary 6beta-OHF/17-OHCS ratio and the serum carbamazepine concentration, corrected for dose and body weight, was examined. RESULTS Serum carbamazepine was inversely associated with the urinary 6beta-OHF/17-OHCS ratio, and the hyperbolic relationship between the two parameters was statistically significant (P < 0.01). DISCUSSION Carbamazepine is well known as a potent inducer and a substrate of hepatic CYP3A4. The present results suggest that measurement of the urinary 6betaOHF/17-OHCS ratio is helpful for assessing individuals' hepatic CYP3A4 capacity after enzyme induction.