Olanzapine steady-state serum concentrations of 334 patients were measured by high pressure liquid chromatography. The patients were subjected to routine therapeutic drug monitoring of antipsychotic treatment. In cases with more than one serum determination per patient, only the first measurement was included. Eighty-four percent of the patients received daily doses in the recommended range from 5 to 20 mg. The interval 25-140 nmol/1 covered about 80% of the concentrations observed for the daily dose range 5-20 mg. This interval may be used as a reference for therapeutic drug monitoring of olanzapine, indicating whether a patient has a typical or an atypical concentration level at a given dose. Such a judgment may be helpful in situations characterized by side effects, insufficient effect, doubts about compliance or possible pharmacokinetic interactions.