We report a 22-year-old man with a solid mass in the right proximal leg, which was furned out to be a gossypiboma. MR imaging revealed a well-defined mass lesion that showed intermediate signal intensity at T1-weighted imaging (T1WI) and slightly high signal intensity at T2-weighted imaging (T2WI). Wavy, low-signal-intensity stripes were visible within the fluid-filled central cavity. At surgical exploration, a sponge, retained after previous knee surgery, was discovered, and it was found that a granuloma had developed. Pathologic examination revealed granulomatous inflammation, with lymphocyte and giant cell infiltration. The presence of wavy, low-signal-intensity gauze fibers at T2WI may be a characteristic MR appearance of gossypiboma.