The present study aims to examine: a) whether pain sensitivity and tolerance are related to heroin dependence and/or treatment outcome and b) whether gene variants are associated with heroin dependence and/or treatment outcome Male Chinese subjects are divided into either heroin-dependent subjects (fulfilled DSM-IV criteria) who have abstained from heroin for one year or more after treatment (no-relapse group) or those who are on methadone or heroin for more than one year and have a history of relapse (relapse group). Healthy male subjects with no history of substance use were recruited as the control group (controls). Cold-pressor test was administered to all participants to ascertain both pain threshold and tolerance. All subjects were genotyped for the functional 孯pioid receptor (MOR) A118G single nucleotide polymorphism. Preliminary data showed that control subjects have the lowest pain threshold and highest pain tolerance when compared to the relapse and no-relapse groups. It is also shown that individuals with the MOR A allele have a lower pain tolerance than those with the G allele in all groups studied. Furthermore, in the relapse group, GG individuals have a significantly higher pain tolerance than the AA individuals (p<0.05). In can be concluded that, firstly, there is an increased pain sensitivity in heroin-dependent subjects (both relapse and no-relapse groups), irrespective of length of abstinence and that heroin-dependence also lowers pain tolerance, a phenomenon also not restored due to abstinence. Secondly, heroin-dependent individuals (both relapse and no-relapse groups) with the MOR G variant have a higher pain tolerance than those with the A allele. This suggests the G allele may be a predictor for heroin-dependence and pain tolerance