A very rare case of fibro-osteochondroma arising in uterine musculature was studied histologically. The tumor exhibited three types of cellular proliferation--fibroblastic, cartilaginous, and osseous--all within an abundant collagenous matrix. The three components were intermingled, and varied in the degree of differentiation of cartilaginous and osseous tissue. The more differentiated tissues were usually circumscribed by fibrocellular bands of spindle-shaped cells that seemed to have transformed into rounded cartilaginous cells. Presumably this tumor arose from undifferentiated mesenchymal cells in the myometrium, which proliferated to form fibroblasts, chondrocytes, and osteoblasts. The tumor may be designated fibrio-osteochondroma. It is histologically benign and exceedingly rare.