The technique of the posterior medial internal sphincterotomy according to Arnous is described and advice from personal experience reported.From 1981 to 1995 posterior medial internal sphincterotomy with anoplasty was performed in 270 patients (132 males and 138 females) affected with chronic anal fissure, alone (112 cases) or associated with hemorrhoids (80 cases); moreover, the Arnous's operation was performed as well for hemorrhoids with posterior subsidiary packet and internal sphincter hypertonia (52 cases), relapsed hemorrhoids with postoperative anal stenosis (24 cases) and postoperative anal stenosis with painful fissure and residual internal sphincter hypertonia (2 cases).The results have been excellent; the average stay in hospital has been 7 days and the complete recovery occurred after 4-6 weeks without early or late complications.The importance of anorectal manometry for the preoperative valuation of the sphincteric hypertonia is emphasized: in this manner it is possible to modulate the sphincterotomy avoiding too economic sphincteric sections with next residue hypertonia or, in the contrary, too plentiful sphincteric section with problems of continence. Finally, the internal lateral-left sphincterotomy is mentioned, which is efficacious in the treatment of acute anal fissure. However, the proctological surgeon, on the basis of his experience, will propose the most convenient technique.