Extended-release naltrexone (XR-NTX) is a once-a-month injectable formulation that is Food and Drug Administration-approved for the treatment of alcohol dependence in patients able to abstain from alcohol before treatment initiation. This paper presents the results of an analysis of efficacy data from a subgroup of patients with 4 days or more of voluntary abstinence before treatment initiation (n = 82) on a wide range of drinking-related outcomes. In these patients, all of whom received counseling, the rate of abstinence was severalfold higher for XR-NTX 380 mg compared with placebo: median time to first drink was 41 days versus 12 days, respectively; rate of continuous abstinence at end of the study was 32% versus 11% (P = 0.02). Extended-release naltrexone 380 mg, compared with placebo, substantially increased time to first heavy drinking event (>180 days vs 20 days; P = 0.04) and decreased the median number of any drinking days per month by 90% (0.7 vs 7.2; P = 0.005) and heavy drinking days per month by 93% (0.2 days vs 2.9 days; P = 0.007). The XR-NTX 380 mg group also had more than twice as many responders compared with placebo (70% vs 30%; P = 0.006; responder defined as having no more than 2 heavy drinking days in any consecutive 28-day period) and experienced greater improvement in gamma-glutamyl transpeptidase levels (P = 0.03). Outcomes for XR-NTX 190 mg (n = 26) were generally intermediate, demonstrating a dose-response effect. In conclusion, XR-NTX 380 mg prolonged abstinence and reduced the number of heavy drinking days and drinking days in patients who were abstinent for as few as 4 days before treatment initiation.