We report a case of sudden onset of deep peroneal neuropathy resulting from a ganglion cyst. Electrophysiology demonstrated a severe deep peroneal neuropathy with axonal loss and probably proximal conduction block. Magnetic resonance imaging demonstrated a mass arising from the proximal tibiofibular joint extending along the peroneal nerve. Surgical resection of the ganglion resulted in gradual improvement of strength. Peroneal nerve ganglia should be considered in the evaluation of patients with sudden painless foot drop.