Extensive literature has investigated the role of serotonin (5-HT) in the control of central dopamine (DA) systems, and their dysfunction in pathological conditions. 5-HT stimulates local DA release in striatal regions via activation of various receptors including serotonin receptor-3 (5-HT3). Several studies have related polymorphisms (SNPs) in the serotonin receptor-3 (HTR3) genes to be associated with pain modulation and endogenous pain suppression. A few studies suggested a functional role of 5'UTR SNP in the serotonergic receptor HTR3A gene (rs1062613) in the development of chronic pain and Fibromyalgia syndrome (FMS) in particular. Here, we investigated the effect of a 5'UTR SNP in the serotonergic receptor HTR3A gene (rs1062613) on striatal dopamine D2/D3 receptor (DRD2) availability and reward-associated DA release in response to unpredictable monetary rewards in 23 women with FMS and 17 age-matched healthy female controls. Furthermore, we aimed to examine if SNP rs1062613 is associated with thermal pain and pain tolerance thresholds.