Congenital (infantile) fibrosarcoma is a rare soft tissue sarcoma occurring in children aged less than 5 years. We performed cytogenetic analysis on such a tumor, which had an abnormal karyotype: 48,XY, +11, +20. Three other reports of karyotypes in congenital fibrosarcoma describe similar cytogenetic changes, and a specific pattern of trisomies that may prove diagnostic for this tumor is beginning to emerge.