Background and aim: Surgical treatment with lateral internal sphincterotomy (LIS) remains the principal treatment for chronic resistant cases with high degree of success despite persistence or recurrence of anal fissures. The incidence of these unfavorable outcomes has been reported to be related mostly to the height of the sphincterotomy. The aim of this study was to evaluate the effectiveness and safety of full length LIS compared to partial length LIS.   Patients and methods: Between May 2008 and June 2014, we surgically treated 80 patients with chronic anal fissure. Group A had partial length LIS and group B had full length LIS. All the patients were continent preoperative and postoperative data were recorded regarding pain, bleeding, infection, recurrence and postoperative incontinence.   Results: None of our patients in both groups had permanent incontinence to flatus or stools. Temporary incontinence to stool and flatus occurred in 9 patients with full length LIS group and only in 3 patients in the partial length LIS.   Conclusion: Full length LIS is a safe effective procedure for the treatment of resistant chronic anal fissure.