We describe five primary soft tissue tumors that had histologic features identical to intraosseous aneurysmal bone cyst. The tumors occurred in three females and two males, who ranged in age from 8 to 37 years (median 28 years). They arose in the deep soft tissue of the upper extremities, thigh, and groin region and typically presented as a rapidly growing mass; no involvement of the adjacent bones was identified. The tumors ranged in size from 2.5 to 9 cm (median 4 cm). Grossly, they were surrounded by a thin rim of bone and on sectioning had hemorrhagic cystic spaces delineated by fibrous septa. Histologically, the cystic spaces were filled with blood and the fibrous septa were composed of fibroblasts, osteoclast-type giant cells, and woven bone. Cytogenetic analysis of one tumor showed 46,XY,t(17;17)(p13;q12), a result similar to that which has been reported for intraosseous aneurysmal bone cyst. The differential diagnosis includes a variety of bone-forming and giant cell-containing tumors, the most important being extraskeletal osteosarcoma. Follow-up showed that four patients are free of disease 16 months to 10 years after surgery; one tumor regrew after incomplete initial excision, but the patient has been free of disease 16 months after a second operation. Soft tissue aneurysmal bone cyst is an uncommon benign tumor that can be treated by simple excision, and it should be distinguished from a variety of other reactive and neoplastic processes.