Prescription charts including a prescription for olanzapine were evaluated for 202 patients from 15 National Health Service trusts. In patients prescribed olanzapine for less than six weeks, the mean dose was found to be 12.4mg/day but 56% of patients were prescribed olanzapine as the sole antipsychotic. For patients prescribed olanzapine for longer than six weeks (when dose titration and drug crossovers were assumed to be complete) the mean dose of olanzapine was 15.8mg/day and 64% received olanzapine as the sole antipsychotic. A substantial proportion of patients were prescribed concurrent anticholinergic drugs (but relatively few when on olanzapine alone). Olanzapine was used in doses higher than the standard dose recommended by the manufacturers, often in combination with typical antipsychotics. Such practices increase costs and, at least in the latter case, very probably adversely affect outcome. Studies are needed to evaluate the relative effectiveness of commonly used antipsychotic regimens in naturalistic settings. In the absence of these, olanzapine should be used as the sole antipsychotic and patients should be property assessed on lower doses before dose increases are undertaken.