Oxcarbazepine (OXC) has been licensed for monotherapy and add-on therapy of focal epilepsy in Germany since February 2000. It is chemically related to carbamazepine, and its anticonvulsant effect has been proven in placebo-controlled double blind studies to be comparable to standard antiepileptic drugs such as carbamazepine, valproate, and phenytoin. Patients whose epilepsy is not well controlled with carbamazepine or in whom side effects occur with carbamazepine can be switched to oxcarbazepine overnight, i.e., without a titration phase. In a retrospective analysis on 51 patients with focal epilepsy, the exchange of carbamazepine with oxcarbazepine was investigated. An exchange in a dosage ratio of 1:1-1:1.5 led to a reduction in seizure frequency by more than 50% in 51% of patients. Oxcarbazepine was better tolerated than carbamazepine. During exchange, CNS toxicity occurred more often with high initial dosages and with an exchange ratio of 1:1.5. In patients pretreated with high dosages of carbamazepine, an exchange ratio of 1:1 and rapid subsequent titration to the maximally tolerated dosage may thus be preferable. In 35% (18/51) of patients treated with oxcarbazepine, hyponatremia developed, which was symptomatic in three patients. Sodium levels should be controlled after exchange and if side effects occur. In conclusion, the overnight switch to oxcarbazepine is an attractive option in patients insufficiently controlled by carbamazepine.