Olanzapine therapeutic drug monitoring (TDM) is the measurement of plasma olanzapine to assess adherence and guide dosage. We have audited data from an olanzapine TDM service, 1999-2009. Multiple linear regression analysis was conducted to investigate the contribution of dose, age, sex, body weight, and smoking status to the plasma olanzapine concentration. There were 5856 samples from 3207 patients. The prescribed olanzapine dosage was 2.5 to 95 mg/d. No olanzapine was detected in 6% of samples. For olanzapine dosages of 2.5 to 20 mg/d, only 35% of results were within a suggested target range of 20 to 39 ng/mL. At doses above 20 mg/d, 30% to 59% of results were 60 ng/mL or greater depending on dose band. In patients aged 17 years or younger (92 samples), median plasma olanzapine was higher than that in adult patients at almost all olanzapine doses. Multiple linear regression analysis of results from 627 adults from whom complete data were available showed that dose, smoking status, sex, age, and body weight together explained 24% the variance in plasma olanzapine. Degree of adherence, timing of sample postdose, drug-drug interactions, and pharmacogenetic factors also may have contributed to the observed variance. However, it is clear that female nonsmokers had higher plasma olanzapine concentrations for a given dose than male smokers. Olanzapine TDM is useful in assessing adherence and may have a role in limiting olanzapine dosage to minimize the risk of long-term toxicity.