A case of carbamazepine neurotoxicity following the addition of diltiazem is reported. A patient with previously stable carbamazepine serum concentrations was treated with diltiazem for atrial fibrillation and a rapid ventricular response. After three days on the combined regimen, the patient developed neurological symptoms resulting in hospitalization. After a review of the patient's medication usage and carbamazepine concentrations, an interaction between diltiazem and carbamazepine was suspected. Carbamazepine's elimination appears to have been decreased, resulting in neurotoxicity. The toxicity resolved when the carbamazepine dosage was decreased 62 percent.