Recently, a role for a functional polymorphism within the promoter region of the serotonin transporter gene (5-HTTLPR) in conferring susceptibility to Obsessive Compulsive Disorder (OCD) has been suggested. The aim of this study was to test the hypothesis that allelic variation of the 5-HTTLPR could be associated with OCD susceptibility or influence the drug response in OCD. One hundred and eighty-one OCD patients were recruited; 92 patients underwent a standardized treatment with fluvoxamine. No significant differences in allele/genotype distribution of the 5-HTTLPR were found between 191 controls and OCD. No differences in fluvoxamine response in the three genotypes groups in OCD were found, considering Yale-Brown Obsessive Compulsive Scale (YBOCS) total scores. Nevertheless, a significant time per genotype interaction was found for the YBOCS subtotal compulsion scores. Considering patients without tic disorder co-diagnosis, a significant time per genotype interaction for both YBOCS total scores and compulsion scores was found.