A recent linkage study suggested that a putative locus for suicidal behavior independent of psychiatric disease phenotypes lies at 5' upstream of the micro-opioid receptor (OPRM1) gene. We explored an association between suicide and genetic variations of the OPRM1 using a case-control study of 183 completed suicides and 374 control subjects. We genotyped four single nucleotide polymorphisms (SNPs) including a common A118G SNP. The genotypic and allelic distributions of the A118G SNP were significantly different between the completed suicide and control groups (P = 0.014 and 0.039, respectively). A dominant model analysis of the A118G SNP showed an enhanced association with suicide (P = 0.0041, Odds ratio 0.575) and this significant association was observed with a logistic regression analysis that takes sex and age factors into account (P = 0.021). Our results raise the possibility that the A118G SNP of the OPRM1 gene is associated with suicide.