Personality traits are among the most complex quantitative traits. Certain personality traits are associated with substance dependence (SD); genetic factors may influence both. Associations between opioid receptor (OPR) genes and SD have been reported. This study investigated the relationship between OPR genes and personality traits in a case-control sample. We assessed dimensions of the five-factor model of personality in 556 subjects: 250 with SD [181 European-Americans (EAs) and 69 African-Americans (AAs)] and 306 healthy subjects (266 EAs and 40 AAs). We genotyped 20 OPRM1 markers, 8 OPRD1 markers, and 7 OPRK1 markers, and 38 unlinked ancestry-informative markers in these subjects. The relationships between OPR genes and personality traits were examined using MANCOVA, controlling for gene-gene interaction effects and potential confounders. Associations were decomposed by Roy-Bargmann Stepdown ANCOVA. We found that personality traits were associated as main or interaction effects with the haplotypes, diplotypes, alleles and genotypes at the three OPR genes (0.002 < P < 0.046 from MANCOVA; 0.0004 < P < 0.049 from ANCOVA). Diplotype TTAGGA/TTCAGA at OPRM1 had main effects on Extraversion (P = 0.008), and diplotypes OPRM1(insertion mark)TTCAGA/TTCAGA and OPRD1(insertion mark)CAC/TAC had interaction effects on Openness (P = 0.010) after conservative correction for multiple testing. The present study demonstrates that the genes encoding the mu-, delta-, and kappa-opioid receptors may contribute to variation in personality traits. Further, the three OPR genes have significant interaction effects on personality traits. This work provides additional evidence that personality traits and SD have a partially overlapping genetic basis.