We report a clinically relevant interaction between a new macrolide antibiotic, clarithromycin, and carbamazepine (CBZ). In a patient receiving CBZ monotherapy, 10-day antibiotic treatment increased CBZ concentration despite concomitant CBZ dose reduction and doubled the CBZ concentration/dose ratio. Concentration of the CBZ epoxide (CBZ-E) metabolite was reduced, suggesting that the interaction occurs at a metabolic level.