Two female siblings died within three months after presenting with paravertebral tumors in the first year of life. The pathology of the two tumors was identical and characteristic of a malignant rhabdoid tumor. There were no identifiable tumor patterns within the kindred which have been associated with any hereditary cancer or precancer syndromes. Fibroblasts were cultured from skin biopsies obtained from the second patient and both parents. Assays of growth kinetics associated with cellular transformation revealed that fibroblasts from the affected sibling can be distinguished from those of the parents and age-matched controls by increased in vitro occurrence of tetraploidy. Such evidence suggests that increased in vitro tetraploidy occurring spontaneously in cultured fibroblasts is an expression of a cancer-prone gene. Increased in vitro tetraploidy has previously been demonstrated in some kindreds with heritable colon cancer syndromes, and may extend our understanding of the genetic etiology of some childhood cancers.