PURPOSE:Malignant rhabdoid tumors were first described in the kidney as a rare variant of Wilms' tumor with a “rhabdomyosarcomatoid” pattern and a particularly poor prognosis. Further studies have demonstrated these neoplasms as a distinct clinicopathologic entity. Subsequently, tumors with a similar histologic appearance, demonstrating the “rhabdoid” cells, have been found in a variety of extrarenal sites.METHODS:We report here a case of malignant rhabdoid tumor of the colon studied with selected molecular techniques.RESULTS AND CONCLUSIONS:This tumor demonstrated several unusual findings for malignant rhabdoid tumors of renal or extrarenal sites, including aneuploidy by flow cytometric analysis and a positive nuclear immunohistochemical staining for p53 protein, which suggests presence of p53 gene mutation. DNA analyses, however, failed to demonstrate the presence of point mutation in any of therasfamily genes.