The case of a patient who developed thrombocytopenia during treatment with carbamazepine (CBZ) is described. The platelet count recovered soon after discontinuation of CBZ, Lymphocyte stimulation test with carbamazepine-10, 11-epoxide (CBZ-10, 11-EPOX), a major metabolite of CBZ, was positive, although with CBZ it was negative. These findings suggest that CBZ-10, 11-EPOX was possibly causative in the pathogenesis of CBZ-induced thrombocytopenia in this case.