Olanzapine is an atypical antipsychotic that, in addition to its use in adults, is now indicated for the treatment of schizophrenia, and manic or mixed episodes associated with bipolar I disorder in adolescents aged 13-17 years. In a randomized, double-blind, multicentre, 6-week trial in adolescents aged 13-17 years with schizophrenia, the least squares mean reduction from baseline to 6 weeks in the Brief Psychiatric Rating Scale for Children (BPRS-C) total score (primary endpoint) was significantly greater with olanzapine than with placebo. In a randomized, double-blind, multicentre, 3-week trial in adolescents, aged 13-17 years, with manic or mixed episodes associated with bipolar I disorder, the mean reduction from baseline to 3 weeks in the Adolescent Structured Young Mania Rating Scale (YMRS) total score (primary endpoint) was significantly greater with olanzapine than with placebo. In extensions of each of the pivotal placebo-controlled trials in schizophrenia and bipolar mania, open-label treatment with olanzapine for up to 26 weeks produced significant reductions from baseline to endpoint in BPRS-C and YMRS total scores, respectively. Oral olanzapine was generally well tolerated in adolescents with schizophrenia or bipolar mania. Sedation and weight gain were the most common adverse events in placebo-controlled trials. Extrapyramidal symptoms were reported by 10% of olanzapine recipients compared with 6% of placebo recipients. Olanzapine-treated adolescents were likely to experience greater increases in bodyweight, sedation, blood lipids, serum prolactin and liver transaminase levels than olanzapine-treated adults. Therefore, careful consideration of risk-benefit is recommended before using olanzapine in adolescents.