Sclerosing epithelioid fibrosarcoma (SEF) is a recently described entity. It is a low-grade sarcoma that occurs primarily in the deep soft tissues of the extremities of adults. It may histologically simulate benign lesions such as fibroma and myxoma or malignancies such as sclerosing carcinoma and lymphoma, extraskeletal myxoid chondrosarcoma, clear cell sarcoma of tendons and aponeuroses, and synovial sarcoma, depending on the lesion's cellularity, degree of fibrosis, and amount of myxoid matrix. There are no previously published cytogenetic studies of this tumor. We found the karyotype 40-45,XY,add(9)(p13),add(10)(p11),-12,-13,-18,add(18)(q11),add(20)(q11) in a SEF of a 14-year-old boy, by using chromosome banding. Fluorescence in situ hybridization showed that both the add(10) and the add(18) contained amplified sequences from 12q13 and 12q15, including the HMGIC gene. Chromosome 18 material was present in the add(9) and terminally in the add(10). The karyotype of this case indicates that SEF is unrelated to extraskeletal myxoid chondrosarcoma, clear cell sarcoma, and synovial sarcoma. When compared with the findings in other soft tissue tumors such as well-differentiated liposarcoma and low-grade malignant fibrous histiocytoma, the chromosome banding and in situ hybridization data add support to the notion that SEF is a relatively low grade variant of fibrosarcoma.