We studied 12 cases of a rare primary bone neoplasm--fibrocartilaginous mesenchymoma--including five from the original report on this condition. The seven male and five female patients were 9 to 25 years old. The metaphyses of the long bones were the most common site, with the fibula accounting for a third of all cases. Histologically, the lesion contained spindle cells, bone trabeculae, and islands of cartilage. At least some of the cartilage was in the form of plates that resembled epiphyseal plates. Intralesional excision led to a high rate of recurrence, but no metastasis or death was related to the tumor. Our results indicate that the entity described by Dahlin and co-authors is histologically distinct and that its behavior does not merit the term malignant.