Denzimol, a new anticonvulsant drug, is currently undergoing clinical evaluation. In this paper we report its use in six patients who were also taking carbamazepine and two patients taking phenytoin. There was a striking elevation of serum carbamazepine, carbamazepine-10, 11 epoxide and phenytoin concentrations in all patients on the addition of denzimol therapy. The interaction with carbamazepine is greater in severity than any other reported to date and denzimol's interaction with both carbamazepine and phenytoin is likely to prove of major clinical significance.