BACKGROUND/AIMS: Multidrug resistance 1 (MDR1) gene encodes P-glycoprotein in intestinal epithelium, which serves as a transmembrane efflux pump of various toxins. mdr1 knockout mice develop spontaneous colitis under specific pathogen free conditions. However, it is unclear that C3435T polymorphism of MDR1 is related to ulcerative colitis. Other studies suggest MDR1 may have an important role in colorectal carcinogenesis. Thus, we evaluated whether MDR1 C3435T polymorphism is present in Korean and it is associated with inflammatory bowel disease or colorectal cancer. METHODS: The genotype distributions of the C3435T polymorphism were investigated by PCR-RFLP method in 94 patients with ulcerative colitis, 24 patients with Crohn's disease, 64 patients with colorectal cancer and each of gender-matched controls with equal numbers. RESULTS: There was no significant difference in frequencies of 3435T allele and 3435TT genotype between patients with ulcerative colitis and controls (p=0.443, p=0.194). No significant difference was present in frequencies of 3435T allele and 3435TT genotype between patients with Crohn's disease and controls (p=0.378, p=1.000). There was neither significant difference in frequencies nor 3435T allele or 3435TT genotype between patients with colorectal cancer and controls (p=0.250, p=0.211). C3435T genotype was not associated with the age of onset or other clinical characteristics in patients with ulcerative colitis, Crohn's disease or colorectal cancer. CONCLUSIONS: MDR1 C3435T polymorphism is also present in Korean and the dominant allele is C. However, there is no evidence that C3435T polymorphism of MDRI is associated to inflammatory bowel disease or colorectal cancer in Korean.