This prospective study aimed to examine the relationship between internal anal sphincter (IAS) damage and ‘idiopathic’ faecal incontinence (FI) in 50 consecutive patients, using endoanal ultrasound examination. The external anal sphincter (EAS) was intact on direct and ultrasonic assessment in all patients. IAS damage was defined as complete rupture or attenuation, less than 2 mm thickness in some part of the sphincter. Complete rupture was found in one patient and damage in a further 17 (total 36%). All three nulliparous patients had normal IAS and EAS. IAS damage was only minimally associated with ‘idiopathic’ FI, suggesting IAS per se is unlikely to be a direct cause of FI in the ‘idiopathic’ group of FI patients. Its role might be analogous to the periurethral striated horse-shoe shape muscle in the urethra, which is thought to act as a mucosal sealant.