The purpose of this study was to clarify the neural anatomy of the sinus tarsi. The nerve endings of the synovium in the sinus tarsi were examined. The synovial membrane in the sinus tarsi was excised in 20 patients with sinus tarsi syndrome (20 feet) and in 2 feet from subjects without symptoms (controls). In 15 of the 20 patients and the two controls, the excised synovial membrane was studied histologically with staining by a modified Gairns gold chloride method. Numerous neural elements were observed in the sinus tarsi in all examined synovium. There were abundant free nerve endings and three types of mechanoreceptors: Pacinian corpuscles, Golgi corpuscles, and Ruffini corpuscles. Macroscopic observation and histological examination, using hematoxylin and eosin, in the other 5 patients revealed chronic synovitis in the sinus tarsi. Our findings suggest that the sinus tarsi is not only a talocalcaneal joint space but a source of nociceptive and proprioceptive information on the movement of the foot and ankle. Sinus tarsi syndrome may result from disorders of nociception and proprioception in the foot.