1. The tarsal tunnel syndrome is a clinical entity and is probably more often encountered than is recognised. 2. The cause is unknown but is probably like that responsible for the carpal tunnel syndrome. Experimental evidence suggests that the sensory symptoms in both these conditions are due to localised ischaemia of the nerve within the fibro-osseous tunnels, and that later structural changes are responsible for motor paralysis. 3. Relief of symptoms is complete if decompression of the posterior tibial nerve is performed before the onset of motor involvement.