Sir: Coates (Psychiatric Bulletin, August 2000, 24, 316) recently reported the value of urine testing to provide objective evidence of non-adherence with olanzapine by a patient appearing before a mental health review tribunal. While this test has its value in the entirely non-adherent patient, the method may have its limitations with the more canny patient wishing to mislead the tribunal. The half-life of olanzapine is 32.4 hours (Reference CoatesCoates, 1999), which results in olanzapine being detectable in urine for about 6 days after ingestion. In the absence of quantitative testing patients can take olanzapine on an infrequent basis and still appear adherent. A serious concern is that patients might choose to mislead tribunals about their adherence by providing the evidence of positive tests for the presence of olanzapine in their urine by taking olanzapine on an infrequent non-therapeutic basis, in order to help effect their discharge.