Objective ATP-binding cassette, sub-family B (MDR/TAP), member 1 (ABCB1) is a drug transporter protein expressed on the epithelial cells of the intestine and the endothelial cells of the blood-brain barrier. Intestinal ABCB1 actively transports drugs from the cell membrane and prevents them from entering the blood stream whereas the blood-brain barrier ABCB1 prevents drugs from entering the central nervous system. In this study, we tested whether genetic polymorphisms within the ABCB1 gene are associated with the severity of depression and the effectiveness of the antidepressant, escitalopram (S-CIT), in treating major depressive disorder (MDD). Methods Twenty single nucleotide polymorphisms in the ABCB1 gene were selected and genotyped in 100 MDD patients who had undergone S-CIT treatment continuously for 8 weeks. The serum concentrations of S-CIT and its metabolites (S-desmethylcitalopram and S-didesmethylcitalopram) were then measured at weeks 2, 4, and 8. Results The ABCB1 genotypes of rs1922242 (P=0.0028) and rs1202184 (P=0.0021) showed significant association with the severity of depressive symptoms as assessed by the Hamilton Rating Scale for Depression adjusted with Hamilton Rating Scale for Anxiety. The haplotype block, rs1882478-rs2235048-rs2235047-rs1045642-rs6949448 (from intron 27 to intron 26), of ABCB1 was found strongly associated with the remission rate (global P = 0.003, d.f. = 69) in which haplotype T-T-T-C-C was associated with a slower remission rate on S-CIT treatment (P =0.001). The haplotypes may not be indicators of the severity of depression or anxiety. Conclusion Our findings suggest that single nucleotide polymorphisms in the ABCB1 gene may be indicators of the severity of depression and of the likely S-CIT treatment remission response in MDD. Pharmacogenetics and Genomics 21:163-170.