Alcohol addiction is one of the most common and devastating diseases in the world. Given the tremendous heterogeneity of alcohol-addicted individuals, it is unlikely that one medication will help nearly all patients. Thus, there is a clear need to develop predictors of response to existing medications. Naltrexone is a μ-opioid receptor antagonist, which has been approved in the United States for treatment of alcohol addiction since 1994. It has limited efficacy, in part because of noncompliance, but many patients do not respond despite high levels of compliance. There are reports that a missense single nucleotide polymorphism (rs179919 or A118G) in the μ-opioid receptor gene predicts a favorable response to naltrexone if an individual carries a "G" allele. This work will review the evidence for this hypothesis. The data are promising that the "G" allele predisposes to a beneficial naltrexone response among alcohol-addicted persons, but additional research is needed to prove this hypothesis in prospective clinical trials.