Two hundred and thirty-five patients suffering from newly diagnosed epilepsy were randomly allocated to treatment with either oxcarbazepine or carbamazepine in a double-blind multi-centre study. After a titration phase (between 4 and 8 weeks), the optimal individual dose of trial medication was determined and treatment with that dose was continued for another 48 weeks. The criteria for assessment were: efficacy--seizure frequency, EEG tracings, global evaluation; tolerability--side effects observed by the patient or the investigator, laboratory tests; other assessments--blood pressure and heart rate, carbamazepine and 10,11-dihydro-10-hydroxycarbamazepine trough serum levels. The results of the study showed the following: no significant difference in seizure frequency between oxcarbazepine and carbamazepine; no correlation between the therapeutic effect and the EEG findings in either treatment group; oxcarbazepine caused significantly fewer (P = 0.04) 'severe' side effects than carbamazepine; global evaluation of tolerability demonstrated a trend towards the better tolerability of oxcarbazepine; no correlation was observed between either efficacy or tolerability and the actual serum trough levels of antiepileptic drugs; clinically relevant abnormal laboratory test findings were observed in 2 patients, both on carbamazepine. The authors consider oxcarbazepine to be a valuable alternative to carbamazepine, particularly in patients who develop side effects which prevent optimal seizure control.