Sarcomatoid carcinomas of the colorectum are rare tu- mors that display both malignant epithelial and stromal components. Clinically, they are aggressive tumors with early metastasis. Due to their infrequent occurrence, the pathogenesis is poorly understood. We report a case of a 52-year-old woman who presented with a rectal mass and intermittent hematochezia. Superficial biopsies during colonoscopy revealed a tubulovillous adenoma with high-grade dysplasia. Endoscopic ultra- sonography confirmed an invasive nature of the mass, and deeper biopsies revealed the presence of neoplasm with mixed histological components. The surgically- excised specimen demonstrated the presence of poorly differentiated spindle cells underneath the tubulovillous adenoma and an intermediate stage of invasive adeno- carcinoma. Based on the histological appearance and immunohistochemical studies, a diagnosis of sarcoma- toid carcinoma was made. Only nine cases of sarcoma- toid carcinomas of the colorectum have been reported to date. As a result, the terminology and pathogenesis of sarcomatoid carcinoma remain speculative. To the best of our knowledge, this is the first report of co- existence of sarcomatoid carcinoma and invasive ad- enocarcinoma with tubulovillous adenoma; all stages represented within the same tumor. This observation supports the “monoclonal theory” of pathogenesis with an adenoma-sarcoma progression with or without an intermediate stage of carcinoma. Sarcomatoid carcinomas of the colorectum are rare tu- mors that display both malignant epithelial and stromal components. Clinically, they are aggressive tumors with early metastasis. Due to their infrequent occurrence, the pathogenesis is poorly understood. We report a case of a 52-year -old woman who presented with a rectal mass and intermittent hematochezia. Superficial biopsies during colonoscopy revealed a tubulovillous adenoma with high-grade dysplasia. Endoscopic ultra-sonography an invasive nature of the mass, and deeper biopsies revealed the presence of neoplasm with mixed histological components. The surgically-excised specimen presenting the presence of poorly differentiated spindle cells underneath the tubulovillous adenoma and an intermediate stage of invasive adeno- carcinoma. Based on the histological appearance and immunohistochemical studies, a diagnosis of sarcoma-toid carcinoma was made. cases of sarcoma- toid carcinomas of the colorectum have been reported to date. As a result, the terminology and pathogenesis of sarcomatoid carcinoma remain speculative. To the best of our knowledge, this is the first report of co- existence of sarcomatoid carcinoma and invasive ad- enocarcinoma with tubulovillous adenoma; all ... This observation supports “monoclonal theory ” of pathogenesis with an adenoma-sarcoma progression with or without an intermediate stage of carcinoma.