Olanzapine is an atypical neuroleptic drug with mood-stabilising properties and few of the side effects commonly associated with conventional neuroleptic treatment. We used olanzapine, 5-20 mg/day, to treat severe aggression in six non-psychotic teenage boys with neuropsychiatric disorders. All but one started to respond within one week. The therapeutic effect in four of the patients clearly outweighed the side effects (weight gain and sedation). The subjects described a markedly increased sense of well being during the olanzapine treatment.