Clozapine is known to induce epileptic seizures and changes in EEG-patterns, including slowing and the appearance of epileptiform activity. Olanzapine, a new antipsychotic drug, shares many pharmacological and clinical properties with clozapine. However, in patients treated with olanzapine, no case of seizure induction has been reported so far, and the EEG has not been studied systematically. We examined the EEGs of patients with schizophrenia treated with either olanzapine (N = 9) or clozapine (N = 9) prior to medication and 3 to 7 weeks afterwards. Clozapine induced significant EEG slowing present in 78% of the patients, and definite epileptiform activity appeared in 33%. Olanzapine also induced significant EEG slowing, but less frequently (in 44% of the patients) and less pronounced than clozapine. Olanzapine had no significant effect an epileptiform activity, but in one patient, an isolated sharp/slow-wave complex was observed. These preliminary data suggest that olanzapine induces EEG slowing to a lower extent than clozapine. Olanzapine's possible effect an the seizure threshold deserves further attention.