BACKGROUND: Extended-release naltrexone (XR-NTX, Vivitrol; Alkermes Inc.) is an injectable monthly sustained-release mu opioid receptor antagonist. XR-NTX is a potentially effective intervention for opioid use disorders and as relapse prevention among criminal justice system (CJS) populations. METHODS: This 5-site open-label randomized controlled effectiveness trial examines whether XR-NTX reduces opioid relapse compared with treatment as usual (TAU) among community dwelling, non-incarcerated volunteers with current or recent CJS involvement. The XR-NTX arm receives 6 monthly XR-NTX injections at Medical Management visits; the TAU group receives referrals to available community treatment options. Assessments occur every 2 weeks during a 24-week treatment phase and at 12- and 18-month follow-ups. The primary outcome is a relapse event, defined as either self-report or urine toxicology evidence of >/=10 days of opioid use in a 28-day (4 week) period, with a positive or missing urine test counted as 5 days of opioid use. RESULTS: We describe the rationale, specific aims, and design of the study. Alternative design considerations and extensive secondary aims and outcomes are discussed. CONCLUSIONS: XR-NTX is a potentially important treatment and relapse prevention option among persons with opioid dependence and CJS involvement. ClinicalTrials.gov: NCT00781898.