Increased pain sensitivity after opioid administration (opioid-induced hyperalgesia) and/or repeated painful stimuli is an individually varying and clinically important phenomenon. The functional polymorphism (val(158)met) of the Catechol-O-methyltransferase (COMT) gene regulates the metabolism of dopamine/noradrenaline. Individuals homozygous for the met(158) allele have been reported to have increased pain sensitivity and there are findings of lower micro-opioid system activation during sustained pain. We hypothesized that met/met individuals would exhibit higher pain sensitization and opioid-induced hyperalgesia in response to repeated pain stimuli and an intravenous injection of an opioid drug. Participants were 43 healthy subjects who went through an experiment where five blocks of pain were induced to the hand using a heat probe. After each stimulus subjects rated the pain on a visual analogue scale (VAS) from 0 mm (no pain) to 100 mm (worst possible pain). Before the second stimulus there was an intravenous injection of a rapid and potent opioid drug. At baseline there was no difference in pain ratings between the COMTval(158)met genotypes, F(2, 39)<1. However, a repeated measures ANOVA for all five stimuli revealed a main effect for COMTval(158)met genotype, F(2, 36) = 4.17, p = 0.024. Met/met individuals reported significantly more pain compared to val/val, p = 0.010. A pairwise comparison of baseline and the opioid intervention demonstrated that analgesia was induced in all groups (p = 0.042) without a separating effect for genotype (n.s). We suggest that the initial response of the descending pain system is not influenced by the COMTval(158)met polymorphism but when the system is challenged the difference is revealed. An important clinical implication of this may be that the COMTval(158)met related differences may be more expressed in individuals where the inhibitory system is already challenged and sensitive, e.g. chronic pain patients. This has to be proven in future studies where the impact of the COMTval(158)met polymorphism on opioid treatment in patients is addressed.