The pelvic girdle, thigh, and calf muscles of 29 patients with Duchenne muscular dystrophy (DMD) were evaluated with magnetic resonance (MR) imaging. The muscles that were most resistant to disease were the gracilis, followed by the sartorius, semitendinous, and semimembranous. Sixteen patients had asymmetric involvement in part of the muscle bundles of thigh. An MR grading system was developed to assess the number of preserved pelvic and thigh muscles, the severity of fatty infiltration of the calf, and the increase of subcutaneous fat. Statistically significant correlations were obtained between MR grade and clinical functional grade (P < .01), MR grade and disease duration (P < .01), and MR grade and patient age (P < .01). An inverse correlation was observed between the creatine kinase values and the MR grade (P < .05). The MR grading system helped identify disease severity in patients with low clinical functional grades. Twenty-four follow-up studies were performed in 22 patients. In 13 of the 24 cases, the MR grade progressed while the clinical functional grade remained unchanged. Both the MR and clinical functional grades progressed in six cases. The results suggest that MR imaging may be useful in prebiopsy mapping and may help accurately monitor the progression of DMD.