Antipsychotic non-adherence rates of outpatients with schizophrenia or schizoaffective disorder was assessed by electronic monitoring and clinician rating. Antipsychotic adherence was determined monthly over 3 consecutive months with (1) the Medication Event Monitoring System (MEMS) cap and (2) the Clinician Rating Scale. Non-adherence was defined as daily adherence of <70% during any one of three monthly evaluations for MEMS and ratings of <or=4 (scale of 1-7) on the Clinician Rating Scale. Non-adherence was detected in 12 of 25 patients (48%) by MEMS and 0% by the Clinician Rating Scale. Clinician assessment dramatically underestimated antipsychotic non-adherence.