Abstract: Most of the posterior complex anal fistulas are ischiorectal fistulas.It is said that the operations for the ischiorectal fistula are very difficult, because of the high recurrence rate and the high incidence of post-operative anal dysfunction.Previously, the ischiorectal fistula was believed to arise from the Cortneyʼs space (deep post anal space) and the secondary ducts to proceed into the ischiorectal fossa.But recently, I discovered new findings regarding the origin of the ischiorectal fistula.These findings are now well accepted by the proctology community in Japan, and I published them in Dis Colon Rectum (2006; 49: S37-S44) .The new findings are as follows.The primary lesion of the ischiorectal fistula is situated in the posterior deep space (PDS) which has been newly defined by my study: the anterior border is the internal sphincter, the superior border is the inferior surface of the puborectalis, the inferior and lateral border is the anterior surface of the external sphincter.The ischiorectal fossa is separated by a septum into two spaces: the high-and low-ischiorectal spaces.In this new classification, so-called pelvirectal fistula, as diagnosed by the conventional method, is actually the high ischiorectal fistula.The new definition of the ischiorectal fistula enables a clearer understanding of fistula development and standardization of surgical procedures for the ischiorectal fistula.In this presentation, I will show the surgical anatomy posterior to the anal canal which is important for the surgical treatment of the anal fistula.And I will also present the ischiorectal fistula etiology and the surgical procedures for its treatment.