The long form of the dopamine D4 receptor (D4DR) exon III repeat polymorphism has been linked in some but not all studies to impulsive, extravagant and novelty-seeking personality traits that are prominent in affiliated behaviours such as attention deficit disorder and substance abuse. In particular, we have reported previously an increased frequency of the long seven-repeat D4DR exon III allele in a group of 141 opioiddependent subjects compared to 110 control subjects. In order to further substantiate the role of D4DR in contributing to heroin addiction we have genotyped an additional, smaller cohort of opioid-dependent subjects. In this new group of 57 opioid-dependent subjects compared to an expanded group of 143 control subjects a significant difference was observed in overall genotype frequency (p=0.04). An excess of the seven-repeat allele of the D4DR receptor gene was also observed compared to control subjects (p=0.06). The frequency of the seven-repeat allele is 15.8% in the heroin addict population vs. 8.1% in the control group, conferring a relative risk of 2.07 (95% CI: 0.98-4.38). An association between two polymorphisms considered together (D4DR and dopamine D3 receptor) and treatment retention was observed (p=0.02). In a subgroup of 38 opioid-dependent subjects, who were successfully administered the TPQ, higher Harm Avoidance (p< 0.001) and Novelty Seeking (NS3; extravagant vs. reserved, p< 0.001) scores were found. In contrast to some previous reports, no relationship was apparent between TPQ scores and treatment retention in this small group of opioid-dependent subjects.