The outcome in treatment of schizophrenia is still not satisfactorily, and using the adjunctive administration of various anticonvulsant drugs adjunctive to antipsychotics has become widely distributed. This study determines the efficacy of oxcarbazepine combined to olanzapine in treatment of schizophrenia in a double-blind, randomized, placebo-controlled, parallel-group, add-on therapy, 7 week study in 54 patients suffering schizophreniform disorder or schizophrenia. Patients were randomized to oxcarbazepine or placebo and titrated up to 1800 mg/ day in week 1 and maintained at that dose for another 6 weeks. Treatment of olanzapine started at week 2 with 5 mg/day. According to weekly improvement in Brief Psychiatric Rating Scale (BPRS), olanzapine dose was maintained constant or escalated in regular steps of 2.5 mg. Main outcome measure was the cumulative olanzapine dose from beginning administration of oxcarbazepine/placebo for a period of 42 days. Comparing treatment of oxcarbazepine and olanzapine with placebo and olanzapine, there was no difference in cumulative olanzapine doses in both groups. in the oxcarbazepine group was not significantly more rescue medication given. A mixed regression model was used to assess time trends in BPRS over the treatment period: the differences in the rate of change of BPRS in the two treatment groups suggested that the scores sank more rapidly in the oxcarbazepine group (p=0.063). Mean post-treatment aggression score also showed no significant difference. Results from this study do not support the use of OXC as an adjunct to atypical antipsychotics in patients with schizophrenia.