CONTEXT: Naltrexone hydrochloride treatment for alcohol dependence works for some individuals but not for everyone. Asn40Asp, a functional polymorphism of the mu-opioid receptor gene (OPRM1), might predict naltrexone response. OBJECTIVE: To evaluate whether individuals with alcoholism who are heterozygous (Asp40/Asn40) or homozygous (Asp40/Asp40) for the OPRM1 Asp40 allele respond better to naltrexone. DESIGN: Pharmacogenetic analysis conducted between January 1, 2001, and January 31, 2004. SETTING: Eleven academic sites in the COMBINE Study. PARTICIPANTS: Recently abstinent volunteers who met all 3 of the following conditions: (1) DSM-IV criteria for primary alcohol dependence; (2) participation in the COMBINE Study; and (3) availability of DNA. INTERVENTIONS: Alcoholic subjects were treated for 16 weeks with 100 mg of naltrexone hydrochloride (234 Asn40 homozygotes and 67 with at least 1 copy of the Asp40 allele) or placebo (235 Asn40 homozygotes and 68 with at least 1 copy of the Asp40 allele). All participants received medical management (MM) alone or with combined behavioral intervention (CBI). MAIN OUTCOME MEASURES: Time trends in percentage of days abstinent, percentage of heavy drinking days, and rates of good clinical outcome. RESULTS: Alcoholic subjects with an Asp40 allele receiving MM alone (no CBI) had an increased percentage of days abstinent (P = .07) and a decreased percentage of heavy drinking days (P = .04) if treated with naltrexone vs placebo, while those with the Asn40/Asn40 genotype showed no medication differences. If treated with MM alone and naltrexone, 87.1% of Asp40 carriers had a good clinical outcome, compared with only 54.8% of individuals with the Asn40/Asn40 genotype (odds ratio, 5.75; confidence interval, 1.88-17.54), while, if treated with placebo, 48.6% of Asp40 carriers and 54.0% of individuals with the Asn40/Asn40 genotype had a good clinical outcome (interaction between medication and genotype, P = .005). No gene x medication interactions were observed in those treated with both MM and CBI. CONCLUSIONS: These results confirm and extend the observation that the functionally significant OPRM1 Asp40 allele predicts naltrexone treatment response in alcoholic individuals. This relationship might be obscured, however, by other efficacious treatments. OPRM1 genotyping in alcoholic individuals might be useful to assist in selecting treatment options. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT00006206.