The 5-HTT gene is a candidate gene for influencing the clinical response to antidepressant treatment. The purpose of this gene study was to determine the relationship between serotonin transporter gene polymorphism at the SLC6A4 and the response to citalopram in a Korean population with major depressive disorder (MDD). Citalopram was administered for 8 weeks to the 80 patients who completed this study. The severity of depression was assessed with the 21-item Hamilton Depression Rating scale, and the 5-HTTLPR genotypes in the patients were determined using the polymerase chain reaction. Our result did not showed significant differences in, allele, and carrier distribution between the normal group and MDD patients. This study suggest that polymorphism of the 5HTT gene was not associated with citalopram response to MDD in the Korean population.