The tarsal tunnel syndrome (TTS) is an entrapment of the posterior tibial nerve at the ankle, which, like carpal tunnel syndrome, improves with surgery but needs an instrumental diagnosis to exclude other diseases. This study was performed to evaluate the diagnostic value of nerve conduction tests proposed for the diagnosis of TTS. Of the 13 patients who were investigated 12 had secondary unilateral and 1 idiopathic bilateral TTS. One or more neurophysiologic parameters were abnormal in all cases. The diagnostic value of each neurophysiologic parameter was calculated by comparing conduction on the affected side with conduction on the healthy side of each patient with TTS. The accuracies of the sensory nerve action potential and mixed nerve action potential, both after stimulation of the plantar nerves, were almost the same, with sensory nerve action potentials more sensitive and less specific and mixed nerve action potentials less sensitive and more specific. Because from the clinical point of view we must be more afraid of false-positive results, the mixed nerve action potential is recommended for presurgical diagnosis of TTS. Coexistence of mixed nerve and sensory nerve action potential abnormalities, especially if asymmetric, are highly indicative of TTS.