Compliance is a problem in all areas of therapeutic medicine. Methods for its assessment are classified as either indirect or direct. Indirect assessment is based on criteria such as pill counts, questionnaires, and self-reporting; direct methods involve the analytic measurement of the drugs in biologic fluids such as plasma or urine. Drugs taken either therapeutically or recreationally become incorporated into hair. This prospective study investigated the relation between the daily intake of the antiepileptic drug carbamazepine and both its trough plasma and hair concentrations in a highly supervised inpatient population of patients with epilepsy during a period of 6 months. Results showed that although there was a significant variation between patients resulting from the substantial range in the daily intake of carbamazepine (800–2400 mg/day), the intrapatient variation in both trough plasma and hair concentrations during the 6-month period were not significantly different. The mean intrapatient percentage coefficient of variation in total plasma and hair concentrations of carbamazepine was 11.5 ± 4.7 and 15.0 ± 5.2, respectively, both of which were independent of the daily dosage. This relatively small intrapatient variation in hair concentration over time and its close relation to the plasma concentration suggests that hair analysis may be a complementary and useful technique in monitoring drug-taking behavior.