The effect of oxaliplatin against colorectal cancer is superior to that of cisplatin, but the molecular mechanism(s) involved is not clear. We found previously that oxaliplatin, but not cisplatin, was transported by human (h) and rat organic cation transporter 3 (OCT3)/SLC22A3. In the present study, we examined whether hOCT3 was significantly involved in the oxaliplatin-induced cytotoxicity and accumulation of platinum in colorectal cancer. The level of hOCT3 mRNA in the colon was 9.7-fold higher in cancerous than in normal tissues in six Japanese patients (P = 0.0247). In human colorectal cancer-derived cell lines, the mRNA of hOCT3 was highly expressed compared with that of other organic cation transporters. The release of lactate dehydrogenase (LDH) and accumulation of platinum with oxaliplatin treatment were increased in SW480 cells transfected with hOCT3 cDNA compared with empty vector-transfected cells. T84 and SW837 cells, with high levels of hOCT3, released more LDH and accumulated more platinum after oxaliplatin treatment than low hOCT3-expressing cells such as SW480, HCT116, HT29, and Lovo. However, the amount of platinum accumulated after cisplatin treatment did not differ among these six cell lines. The levels of hOCT3 expression in colon and rectum were also higher in cancerous than in normal tissues in Caucasian patients as determined by dot blotting. In conclusion, the hOCT3-mediated uptake of oxaliplatin into the cancers was suggested to be important for its cytotoxicity, and hOCT3 expression may be a marker for cancer chemotherapy including oxaliplatin.