The serotonin 6(5-HT6) receptor gene is a candidate gene for influencing the clinical response to treatment with antidepressants. The purpose of this study was to determine the relationship between the C267T polymorphism in the 5-HT6 receptor gene and the treatment response to citalopram in a Korean population with major depressive disorder(MDD). Methods : Citalopram was administered for 8 weeks to the 90 patients who completed study. 21-item Hamilton depression rating scale(HAMD-21) was used as a outcome measure. Results : We found that the genotype, allele, and allele-carrier distributions did not differ significantly between MDD patients and normal controls. A main effect of an interaction of genotype with time on the decrease in the HAMD-21 score during the 8 weeks study period was not found. ANOVA revealed no significant effects of the C825T polymorphism on the decrease in the HAMD-21 score at each time period. Conclusions : These results suggest that the C267T polymorphism in the 5-HT6 receptor gene is not associated with the treatment response to citalopram.