Because vigabatrin (VGB) is not metabolized by liver enzymes and does not bind with serum proteins, there is little theoretical chance of it interacting with other antiepileptic drugs. However, our observations have shown that if VGB is added to carbamazepine (CBZ) monotherapy, some patients respond with adverse, toxic symptoms suggesting possible carbamazepine-vigabatrin interaction. This article presents the results of a study of 66 epileptic patients (27 women and 39 men), age 10-66 years (mean, 28.2 years), with focal seizure onset with or without secondary generalization. In these patients, in addition to CBZ therapy with an average dose of 16.7 mg/kg per day (8.6-26.8), VGB, average dose 31.1 mg/kg per day (7.1-57.9), was added. CBZ concentration was measured twice: prior to VGB introduction and 5-12 weeks after the final dose of VGB was reached. In our study 69.7% of patients responded to VGB addition with a significant increase (by at least 10%) in CBZ concentration. A correlation between the value of the increase and the initial level of CBZ prior to VGB addition was found also. Correlational analysis (Pearson's r) revealed a negative correlation between CBZ concentration and increased concentration after VGB addition (r = -0.47, df = 64, P < 0.001). This negative correlation means that if the initial CBZ level is lower, its concentration value after VGB addition is higher.