To assess whether magnetic resonance imaging could assist in diagnosing Ewing sarcoma, we retrospectively analyzed T1- and T2-weighted images of 24 consecutive solitary lytic bone lesions, including 18 cases of histologically proven Ewing sarcoma, and six aggressive-appearing benign lesions including eosinophilic granuloma and osteomyelitis (3 cases each). Images were obtained using a (1.0T) Siemens GBSII system. No signal intensity differences were noted among the various lesions on T1-weighted images. The observed differences in T2-weighted marrow signal intensities were scored on a 5-point scale (5 = water, 3 = fat, and 1 = muscle) and then histologically correlated. Sixteen of 18 Ewing sarcomas showed homogeneous T2-weighted signal isointense with fat and two were heterogeneous. Signals for all 6 benign lesions were homogeneous and more intense than fat on T2-weighted images. All Ewing sarcomas had associated soft tissue masses whereas, only one benign lesion, an eosinophilic granuloma had a small soft tissue component. In our patient population, a homogeneous intramedullary T2-weighted signal isointense with fat, and the presence of an associated soft tissue mass helped differentiate Ewing sarcoma from benign lytic lesions. The reproducibility of these results requires testing in larger series of patients using a variety of MR imaging equipment of differing field strengths.