Cytogenetic, immunocytochemical, and ultrastructural studies were performed on a large congenital fibrosarcoma. To our knowledge, this is the first report of a congenital fibrosarcoma characterized by all of these techniques. The findings of immunochemical and electron microscopic studies supported the suggestion that the tumor is of fibroblastic origin. The karyotype of the tumor (48,XY,+11,+20) is similar to that observed in previously reported cases and substantiates the hypothesis that congenital fibrosarcoma is characterized by nonrandom gain of chromosomes.