Electroencephalogram (EEG) slowing is associated with clozapine side effects, e.g., sedation, and may predict treatment response during clozapine treatment. As olanzapine and clozapine share many pharmacological properties, we investigated whether EEG slowing during olanzapine treatment was related to therapy outcome and sleepiness in patients with schizophrenia. Participants were age- and gender-matched schizophrenic patients treated with olanzapine (n 54), receiving no pharmacological treatment (n 54), or cotreated with olanzapine and some other psychotropic drug (n 38). Their EEG recordings were assessed visually by the same rater blind to clinical data. The EEG scores were categorized using standardized forms. Patients with a poor treatment response did not differ significantly from those with a good response to treatment either in EEG patterns or in frequency of sleepiness. Olanzapine treatment was associated with increased rates of slow (70.4% vs. 22.3%) and sharp waves (22.2% vs. 7.4%), as well as of paroxysmal slow wave discharges (14.8% vs. 1.9%), but did not induce spike- or sharp-slow-wave complexes. Cotreatment with another antipsychotic further increased EEG abnormalities, whereas benzodiazepine administration diminished the olanzapine-induced EEG changes. The results show that olanzapine inducing both slow and sharp waves, as well as paroxysmal discharges, has a strong impact on EEG. However, as no spike- or sharp-slow-wave complexes were observed, the risk of epileptic seizure during olanzapine treatment can be regarded as low, as long as olanzapine is not combined with some other antipsychotic.