The sinus tarsi syndrome is now a well-defined entity of foot pathology. Usually post-traumatic, it is characterised by pain over the lateral opening of the sinus tarsi and a feeling of instability of the ankle. We were able to define two objective criteria for the syndrome: arthrography of the subtalar joint shows specific abnormalities and recording of the electromyographic function of the peronei demonstrates abnormalities during walking. Conservative treatment is successful in about two-thirds of the cases. In other cases a simple surgical procedure can cure or improve 90% of the patients.