The selective serotonin re-uptake inhibitors (SSRIs) which modulate serotonergic activity are effective in the treatment of serotonin-related mental disorders, such as depression and anxiety. These agents bind to the serotonin transporter (5-HTT) and inhibit its capacity to transport serotonin (5-hydroxytryptamine; 5-HT). A functional polymorphism in the promoter region of 5-HTT (5-HTTLPR) has been described. The insertion variant of this polymorphism (long allele) is associated with higher expression of brain 5-HTT compared to the deletion variant (short allele). An association between the 5-HTTLPR polymorphism and mental disorders has been reported by some, but not all, investigators. In addition, the 5-HTT gene polymorphisms were found to be associated with a better and faster response to SSRIs with or without pindolol augmentation in depressed patients. Further studies are needed to clarify the relationship between the 5-HTT genotype, the susceptibility to mental disorders, the response to serotonergic agents and the side effect profile.