We describe a case of a proximal anterior leg mass causing weak great toe dorsiflexion. The lesion was demonstrated on MR imaging as a multilocular cystic lesion within the extensor digitorum longus muscle that was connected to the proximal tibio-fibular joint by a pedicle. Fluid extravasation was present that extended inferiorly from the lesion along the fascial planes into the distal anterior lower leg. At surgery, the lesion was found to be compressing the deep peroneal nerve. The final pathologic diagnosis was peroneal ganglion cyst.