A pharmacogenetic analysis based on preliminary evidence showing that a functional polymorphism in the gene encoding the mu-opioid receptor (OPRM1) affects response to naltrexone will serve to explore an important source of variation in the response to naltrexone treatment. Exploratory analyses involving other potential genetic moderators of the response to naltrexone, such as the gene encoding the delta opioid receptor (OPRD1), will also be conducted, as will the correlation of genotype data with other phenotypes.