Heroin's analgesic, euphoric and dependence-producing effects are primarily mediated by the mu opioid receptor (MOR). A single gene, OPRM1, encodes the MOR. The functional polymorphism A(118)G, located in exon 1 of the OPRM1 gene, results in anatomically-specific reductions in MOR expression, which may alter an individual's response to heroin. In prior studies 118G (rare allele) carriers demonstrated significantly greater opioid tolerance, overdose vulnerability, and pain sensitivity than 118AA homozygotes. Those findings suggest OPRM1 genotype may impact characteristics of heroin use.The present pilot study characterized the impact of OPRM1 genotype (rs1799971, 118G allele carriers vs. 118AA homozygotes) on heroin-use phenotypes associated with heroin dependence severity in a sample of male, Caucasian chronic heroin users (n = 86).Results indicate that 118G allele carriers reported significantly more heroin use-related consequences and heroin-quit attempts, and were more likely to have sought treatment for their heroin use than 118AA homozygotes.These preliminary findings, consistent with extant data, illustrate a role for OPRM1 allelic variation on heroin use characteristics, and provide support for considering genotype in heroin treatment and relapse prevention.