Some studies have shown that patients entering buprenorphine treatment differ from those in other modalities. This study compares Massachusetts Medicaid beneficiaries who received buprenorphine, methadone or other treatment for opioid addiction in 2007. Patients' characteristics and comorbidities were identified through claims data, and associations between these factors and treatment type were investigated using multivariate analysis. Among patients receiving opioid agonist treatments, patients with prior buprenorphine treatment, HIV, bipolar disease, and other substance use disorders were more likely to receive buprenorphine treatment compared with methadone, whereas patients with heart failure, diabetes, hepatitis C, major depression, and anxiety were less likely to receive buprenorphine treatment. These differences may suggest variability in patient access, treatment preferences, and a need for different levels of services in different modalities. This information is important for understanding the impact of this new treatment in Medicaid populations and for developing treatment systems to best meet patients' needs.