A prospective comparison of the accuracy of MR and CT in determining the anatomic extent of disease was carried out in 40 patients who had histologically proved soft-tissue sarcomas of the extremities. Tumor resection, performed by using either en bloc excision or amputation in all patients, allowed detailed pathologic verification of imaging findings. Image analysis included measurement of maximum tumor dimensions, notation of tumor position in relation to deep fascia, recording of anatomic compartment and individual muscle involvement, and documentation of the spatial relationships between tumor, neurovascular structures, underlying bones, and joints. MR and CT were equally accurate in measurement of maximal tumor diameter, detection of tumor depth, and delineation of tumor, neurovascular, osseous, and articular relationships. Evaluation of anatomic compartment and individual muscle involvement was more accurately accomplished with MR imaging; nine (23%) of 40 MR studies showed tumor involvement of one or more individual muscles that appeared normal in CT scans. These results suggest that MR imaging may be the staging procedure of choice in patients with soft-tissue sarcomas of the extremities.