In vitro lymphocyte proliferation induced by carbamazepine (CBZ) was evaluated in nine patients with hypersensitivity to this drug. Lymphocytes from all hypersensitive patients responded by significantly enhanced DNA synthesis to CBZ when patients were compared with 33 tested control subjects. However, lymphocytes from five of six hypersensitive patients were not stimulated by carbamazepine-10, 11-epoxide, and oxcarbazepine (OXC), and this was confirmed clinically in two CBZ hypersensitive patients with OXC therapy. The results indicate that OXC may be suitable as an alternative therapy in some patients with CBZ hypersensitivity.