Background: Carbamazepine (CBZ) is one of the frequently prescribed drugs for partial or generalized epilepsy and its metabolite carbamazepine epoxide(CBZE) is known to have therapeutic effect and toxicity. To evaluate its metabolic pattern and influence of other antiepileptics in Korean patients, we measured the plasma concentrations of CBZ and CBZE in patients undergoing CBZ therapy with fixed doses more than three months. Methods : Blood samples from 110 adult patients on maintenance CBZ therapy with normal liver and kidney function were collected within the last three hours of their dose intervals. Total and free plasma concentration of CBZ and CBZE were measured by HPLC and factors influencing them were statistically persued. Results : CBZ and CBZE concentrations in 110 patients were distributed from 0.12 to 17.7 ${mu}g/ml$, and $0{sim}6.53{mu}g/ml$, respectively. Free fractions of CBZ and CBZE were not influenced by dose or coadministration. Plasma concentration of CBZ and CBZE were significantly correlated with daily CBZ dose regardless of poly therapy. But the relationship between the concentration ratio and CBZ dose was significant only in monotherapy group. Poly therapy significantly increased plasma CBZE concentration and CBZE/CBZ ratio. The increase in CBZE/CBZ ratio was more evident than that of CBZE concentration itself. Those who were coadministered with valproate showed higher CBZE/CBZ ratio than monotherapy group. Conclusion: Coadministration of other antiepileptics with CBZ significantly increased the metabolic elimination of CBZ to its epoxide metabolite and the higher epoxide concentration observed in the poly therapy group makes it necessary for the clinicians to carefully monitor its adverse reactions even when the parent drug levels are in therapeutic range.