Objective: to assess the efficiency of endorectal ultrasound (ERUS) in the study of chronic idiopathic anal pain (CIAP).Material and method: this is a prospective and descriptive study in which 40 patients, 18 men and 22 women with an average of 47 years, were included. They had chronic anal pain of at least 3 months' duration. A complete colonoscopy was performed in all patients, which found no abnormalities to explain clinical symptoms. Patients with anal fissure and internal hemorrhoids of any degree, perianal suppurative processes, and pelvic surgery were excluded from the study. An ALOKA ProSound SSD-4000 ultrasound console attached to a multifrequency radial transductor ASU-67 (7.5 and 10 MHz) was used.Results: one patient could not tolerate the examination. In 8 patients (20% of cases) alterations were detected during ultrasonography: in 4 patients (10% of the cases; 1 man and 3 women) internal anal sphincter (IAS) hypertrophy, and in 5 patients (4 women and 1 man) a torn sphincter complex. A tear in the upper IAS canal and hypertrophy of the middle anal canal were observed in one patient (1 woman).Conclusions: ERUS is a simple, economic and useful test to study anorectal pathologies. Although in most studied cases no damage to the anal canal or rectal wall was detected, in a considerable number of patients we observed a thickening of the IAS, a probable cause of anal pain. Therefore, we understand that ERUS should be included in the study of CIAP.