The malignant rhabdoid tumor is a rare, poorly understood tumor which occurs primarily in children. The kidney is a frequent primary site of origin, but the tumor has arisen in other mesodermally derived tissues as well. Controversy exists regarding the embryonic origin of the rhabdoid tumor and recent histopathologic studies suggest that it may be of neuroepithelial origin. Our immunohistochemical and electron micrographic studies support this theory. No consistent chromosome abnormalities have been reported in this tumor and no cell lines are available for study. We have established and characterized the first rhabdoid tumor cell line. It possesses a specific chromosomal abnormality, 46,XY,t(11;22)(p15.5;q11.23). The translocation may provide an important clue to the pathogenesis of the tumor as well as an opportunity for further study of the involved chromosome regions.