MR imaging was performed in eight patients with osteochondromas, and the results were compared with the histologic findings in six of these cases. MR imaging effectively showed the essential anatomic features of lesions and was particularly useful in assessing the presence and thickness of the cartilage cap. The cartilage cap was clearly seen as a region of high signal intensity on T2-weighted images. A superficial zone of low signal intensity was seen covering the cartilage cap, corresponding to an intact perichondrium, in all of the five cases in which histologic examination confirmed the presence of an intact perichondrium. The results of MR in these cases suggest that MR is useful in the preoperative planning of cases in which excision of an osteochondroma is necessary. Optimal surgical resection of the lesion requires complete excision of an intact perichondrium to prevent possible recurrence of the tumor.