This study assessed the utility of adding the Addiction Severity Index (ASI) to demographic and clinical diagnostic information for the purpose of predicting subsequent substance use disorder service use, and use of other healthcare services by 260 veterans admitted for outpatient substance use disorder treatment. Data collected included demographics, clinical diagnoses, assessment data from the ASI, as well as measures of six-month health service utilization (e.g., substance use disorder services, other mental health services, outpatient medical visits, urgent care visits, inpatient psychiatric and medical). Multivariate analysis using Tobit regression models showed six out of seven ASI scales were significant predictors, and that combining ASI data with demographics and clinical data significantly improved prediction of health care services. It also was found that certain psychiatric and medical diagnoses were related to service use measures, and that a diagnosis of depression was related to overall healthcare utilization.