We report an unusual case of a woman who sought medical attention for a painful mass within her anterolateral left thigh. The patient's symptoms waxed and waned with her menses. Magnetic resonance imaging revealed a 3 cm mass isointense to muscle and surrounded by a rim of decreased signal intensity on T1-weighted images. T2-weighted images with fat saturation revealed a low signal intensity mass with a surrounding rind of high signal intensity edema. Based on its magnetic resonance imaging characteristics, the lesion was initially thought to represent the sequela of prior trauma or perhaps a small fibrous tumor. An excisional biopsy of the mass, however, revealed endometrial tissue, and a pathologic diagnosis of intramuscular endometriosis was rendered.