Hair samples were obtained from 30 patients who had been taking carbamazepine in fixed daily doses for more than 6 months. Carbamazepine was extracted from the hair by enzymatic hydrolysis and quantified by gas chromatography-mass spectrometry. Concentrations ranged from 1.2 to 57.4 ng mg-1. The concentrations of carbamazepine were significantly correlated (P < 0.0001) with the daily dose and the correlation coefficient was 0.793. Although it had been suggested by several authors that the measurement of carbamazepine in hair might provide a better index of individual dosage history than the plasma level assays, the deviations observed in this study led to the conclusion that hair samples are not suitable for evaluating the quantity of drug consumed. However, hair testing may be useful for documenting clinical disorders by sectioning the shaft into segments corresponding to 1 month periods.