Deep peroneal nerve mononeuropathy is rarely observed in clinical practice. We describe a case in which the syndrome was caused by a clinically not detectable ganglion within the anterior compartment of the leg. Electrophysiological data allowed us to localize the site of the lesion for a correct planning of CT scan. CT scan showed a low density tissue occupying the anterior compartment of the leg and its relationships to muscles, septa, bones and vessels. Electrophysiological data and CT scan utilities are discussed.