To determine the frequency and clinical significance of tumor-associated muscle edema, magnetic resonance (MR) imaging findings in 46 consecutive patients with benign or malignant musculoskeletal lesions were reviewed. Increased muscle signal intensity on T2-weighted, STIR (short-inversion-time inversion-recovery), and gadopentetate dimeglumine-enhanced T1-weighted images was present in 41 cases, with the clearest delineation of tumor margins seen on T2-weighted images. Typical peri/paratumoral edema (PTE) was present in equal proportions of malignant (25 of 37) and benign lesions (six of nine). Massive edema involving the entirety of at least one contiguous muscle--to the authors' knowledge, a previously undescribed finding--was identified on MR images of eight malignant and two benign lesions (22% of both groups). All cases of massive edema had a substantial soft-tissue component and involved muscles disrupted by tumor at the point of attachment to bone. Malignant tumors associated with massive edema were larger than those with typical or no PTE, showed a poorer response to initial chemotherapy, and had a higher frequency of metastases at diagnosis. Thus, the presence of massive muscle edema appears to be an ominous clinical finding in patients with malignant musculoskeletal lesions.