Among outpatients with schizophrenia, antipsychotic non-adherence is common, grossly under-detected by patients and their prescribers, and is associated with poor clinical outcomes. Using electronic monitoring (EM) as the reference standard we evaluated the reliability and validity as well as the sensitivity and specificity of a recently developed, brief, pencil-paper, clinician-administered adherence instrument [the Brief Adherence Rating Scale (BARS)] to assess the oral antipsychotic medication adherence of outpatients with schizophrenia and schizoaffective disorder. EM and BARS adherence and symptom severity ratings were gathered at baseline and prospectively at 6 monthly visits in 61 participants (n=35 with schizophrenia; n=26 with schizoaffective disorder). A significant positive relationship was found between mean BARS and EM adherence (beta=0.98; rs=0.59, p<0.0001). Cronbach's coefficient alpha revealed very high internal reliability for the BARS (alpha=0.92). A moderate-to-strong degree of test-retest reliability was also found for the BARS (beta ranged from 0.53 to 0.92 and rs ranged from 0.46 to 0.86). Regarding concurrent validity of the BARS, greater mean BARS adherence was significantly related to lower mean PANSS total scores (beta=-0.40; rs=-0.39, p=0.002) and to lower mean Positive symptom sub-scale scores (beta=-0.08, p=.007; rs=-0.28, p=.02). An initial 3-month monitoring period with the BARS also demonstrated good sensitivity (73%) and specificity (74%) in identifying non-adherent outpatients (defined as <70% mean EM adherence). Relative to EM, the BARS appears to provide valid, reliable, sensitive, and specific estimates of antipsychotic medication adherence of outpatients with schizophrenia and schizoaffective disorder. The BARS appears to be a promising candidate as a brief adherence assessment instrument for feasible use in community-based settings.