Background: Orthotopic mouse models of human colorectal cancer represent an important in vivo tool for testing chemotherapeutic agents and studying intraluminal factors that may alter the growth of cancers. Currently the orthotopic mouse models of colorectal cancer require either an operative procedure or creation of colitis to implant the cancer cells in rectum. We have developed a nonoperative, minimally invasive technique to create a true orthotopic colon cancer mouse model. Study Design: We used human (LS 174T and HT-29) and murine (CRL-2638 and CRL-2639) colon cancer cell lines. Low-dose mucosal coagulation was performed transanally using a specially designed electrode at 1 to 3 predetermined points, 2 to 3 cm from the anus, followed by a transanal instillation of tumor cells (1 × 106 cells) in 12 female nude or severe combined immunodeficiency disease mice for each of the 4 groups (n = 48, plus 16 controls). Mouse colonoscope (Coloview) and microCT were used to follow tumor growth. Four mice from each group were euthanized at 1, 2, and 3 weeks. Results: Tumors were detected in 12 of 12 of the CRL-2638, 11 of 12 of the CRL-2639, 7 of 12 of the HT-29 and 12 of 12 mice in the LS 174T groups. Histopathologic evaluation showed that the tumors grew from the colonic mucosa. CRL-2638 and LS 174T exhibited better implantation, faster tumor growth, larger tumor volume, and earlier metastases. MicroCT detected tumors larger than 3 mm, and the colonoscopy detected tumors larger than 1 mm. Conclusions: Our mouse model is minimally invasive and easy to create and overcomes the limitations of existing models while mimicking the human disease in terms of morphology and biologic behavior. This model opens the doors for colon cancer oncologic studies in an animal model that were previously not possible.