A 27-year-old man presented with a 1-year history of discomfort in the proximal right thigh. Six months after the onset of symptoms, he started to feel a mass in the same area, at the lateral aspect of the proximal right thigh. The mass gradually increased in size. There was no history of trauma and no associated fever, chills, or weight loss.At physical examination, there was a large palpable mass at the anterolateral aspect of the right thigh, occupying the proximal one-third of the thigh. The mass was firm and nonmovable but mildly tender. There were no overlying skin changes or associated inguinal lymphadenopathy.His earlier medical history was noncontributory. Complete blood cell count, erythrocyte sedimentation rate, and results of a basic electrolyte panel were normal.Abbreviations: ASPS = alveolar soft-part sarcoma, EMA = epithelial membrane antigen, PAS = periodic acid-Schiff, STIR = short inversion time inversion-recovery RadioGraphics 2013; 33:585-593 • Published online 10.1148/rg.332115173 • Content Codes: