OBJECTIVE: To determine if a single baseline adherence assessment (Brief Adherence Rating Scale [BARS]) could identify patients who are likely to respond to long-acting injectable (LAI) antipsychotic treatment. METHOD: The current secondary analysis included a sub-sample of adult outpatients (N = 176) with schizophrenia or schizoaffective disorder who participated in the "A Comparison of Long-Acting Injectable Medications for Schizophrenia (ACLAIMS)" trial and had a baseline BARS assessment and a baseline and month 3 Positive and Negative Syndrome Scale (PANSS) rating. The main outcome was LAI treatment response, defined as a ≥ 20% decrease (baseline to month 3) on the PANSS total score. Receiver Operating Characteristic (ROC) and Area Under the Curve (AUC) analysis was conducted to determine the optimal cutpoint of baseline BARS adherence in discriminating LAI treatment response at month 3. A logistic mixed model estimated the odds of response to LAI treatment at month 3 from the optimal baseline BARS cutpoint. RESULTS: The ROC analysis determined that the single baseline BARS rating (cutoff ≤66%), indicating low adherence, best discriminated patients likely to respond to LAI treatment (AUC = 0.603, SE = 0.046, 95% binomial exact CI = 0.527 to 0.676, p = 0.025), with 38% sensitivity and 85% specificity. The logistic mixed model analysis revealed that patients with ≤66% BARS adherence had 3.464 times the predicted odds (95% CI = 1.604 to 7.480, p = 0.001) of responding to LAI treatment than those who were >66% BARS adherent. CONCLUSION: A single baseline BARS assessment discriminated response to LAI treatment suggesting it is a reasonable tool to identify candidates for LAI antipsychotic treatment.