Background and study aims: Outcomes following surgical treatment of patients with anal fistula are related to eradication of tracts and the internal opening. In this study, the results of surgery based on endoanal ultrasound (EAUS) findings were evaluated. Patients and methods: A total of 102 patients with primary cryptogenetic anal fistula were prospectively examined with EAUS, using a 360-degree rotating 10-MHz probe, equipped with a three-dimensional (3-D) imaging system. Injection of hydrogen peroxide through the external opening was also used. Patients underwent operation on the basis of the EAUS findings. The agreement between findings from EAUS and from surgery was calculated. Clinical results were reported as treatment success, fistula recurrence, and fecal incontinence. Results: Amongst 102 patients, the overall concordance between EAUS and surgical findings was 94.1% for primary tracts, 91.2% for internal openings, 96.1% for secondary tracts, 100% for abscesses, and 96.1% for horseshoe tracts. Diagnostic accuracy was improved when hydrogen peroxide injection or 3-D imaging were used. Fistulotomy was performed in 46 patients (45.1% ), fistulectomy in 17 (16.7% ), fistulotomy plus seton placement in 19 (18.6% ), fistulectomy plus seton in 18 (17.6% ), and mucosal flap advancement in 2 (2.0% ). The operation was curative in 100 patients (98.0% ), and unsuccessful in 2 (2.0% ) due to recurrence of the fistula. Fecal continence was preserved in all patients. Conclusions: These data highlight the diagnostic accuracy of EAUS, particularly when hydrogen peroxide injection or 3-D imaging are used. Basing our surgical decision making on EAUS findings allowed us to carry out curative operations in a significantly large number of patients; the recurrence rate was very low. The accurate EAUS assessment of the relationship between fistulas and sphincters has been the main factor in choosing a sphincter-saving surgical procedure, avoiding fecal incontinence. Background and study aims: Outcomes following surgical treatment of patients with anal fistula are related to eradication of tracts and the internal opening. In this study, the results of surgery based on endoanal ultrasound (EAUS) findings were were. of 102 patients with primary cryptogenetic anal fistula were prospectively examined with EAUS, using a 360-degree rotating 10-MHz probe, equipped with a three-dimensional (3-D) imaging system. Injection of hydrogen peroxide through the external opening was also used Patients underwent operation on the basis of the EAUS findings. The agreement between findings from EAUS and from surgery was calculated. The results of findings from EAUS and from surgery was calculated. and surgical findings was 94.1% for primary tracts, 91.2% for internal openings, 96.1% for secondary tracts, 100% for abscesses, and 96.1% for Fistulotomy was performed in 46 patients (45.1%), fistulectomy in 17 (16.7%), fistulotomy plus seton placement in 19 (18.6%), fistulectomy plus seton in 18 (17.6%), and mucosal flap advancement in 2 (2.0%). The operation was curative in 100 patients (98.0%), and unsuccessful in 2 (2.0%) due to recurrence of the fistula. Fecal continence was preserved in all patients. Conclusions: These data highlight the diagnostic accuracy of EAUS, particularly when hydrogen peroxide injection or 3-D imaging are used. Basing our surgical decision making on EAUS findings allowed us to carry out curative operations in a significant large number of the recurrence rate was very low. The accurate EAUS assessment of the relationship between fistulas and sphincters has been the main factor in choosing a sphincter-saving surgical procedure, avoiding fecal incontinence.