BACKGROUND AND AIM: Excessive alcohol use increases mortality and morbidity among opioid substitution therapy (OST) clients. Regular attendance for OST dosing presents key opportunities for screening and treatment. However, individuals' perception of their alcohol consumption as problematic or otherwise may impact their willingness to change. This study examines patterns of alcohol consumption among OST clients, perceptions of their own use and correlates of excess consumption. METHODS: Confidential, structured interviews were conducted with 264 clients of two Sydney OST clinics. Alcohol consumption was assessed using the Alcohol Use Disorders Identification Test (AUDIT); and illicit drug dependence with the Severity of Dependence Scale. RESULTS: Forty-one percent of the participants scored ≥8 on the AUDIT ('AUDIT-positive'), indicating excessive alcohol use. The higher a participant's AUDIT score, the more likely they were to demonstrate insight into the potential problems associated with their drinking (linear trend, p<0.01). However, only half of AUDIT-positive participants believed they drank too much and/or had a problem with alcohol. One-third had discussed their drinking with OST staff, and a similar proportion reported a history of alcohol treatment. AUDIT-positive participants were more likely than others to be classified as dependent on an illicit drug in the last six months (AOR=1.76, 95% CI:1.00-3.09), report a history of alcohol treatment (AOR=5.70, 95% CI:2.83-11.48) and believe it is safe to drink 4+ standard drinks in one session (AOR=5.30, 95% CI:2.79-10.06). CONCLUSIONS: OST clients with AUDIT scores ≥8 appear to underestimate the risks associated with their alcohol consumption. Regular assessments of alcohol use and targeted brief alcohol interventions may improve health outcomes among OST clients.