A case of a small cell malignant tumor that occurred in the soft tissues of a 16-year-old boy with Down syndrome (47,XY,+21) is reported. The histologic and histochemical patterns were consistent with an extraskeletal Ewing's sarcoma (ES). The cytogenetic analysis of the tumor cells showed a t(11;22)(q24;q21), tetrasomy of chromosome 21, and trisomy of chromosome 14. The observation of a t(11;22) in an ES gives credit to the morphologic evidence in favor of the common (probably neuroectodermal) origin of the skeletal and extraskeletal forms of Ewing's sarcoma (ES). The possible pathogenetic significance of the constitutional trisomy of chromosome 21 in determining the occurrence of this tumor is discussed.