PURPOSE:The aim of the study was to evaluate the use of anal endosonography in idiopathic incontinence.METHODS:In 29 patients and 26 normal controls, the relationship between sonography images and physiologic parameters was studied.RESULTS:External anal sphincter function, measured as fiber density by single‐fiber electromyography(P=0.0001) and pudendal nerve terminal motor latency(P=0.04), was significantly impaired in patients with idiopathic incontinence compared with controls. Both the external and internal anal sphincter could be identified by anal endosonography, and the thickness directly measured. The thickness of the external anal sphincter was significantly negatively correlated to muscle fiber density(r=−0.65,P=0.0002) and to pudendal nerve distal conduction velocity(r=−0.74,P=0.008). The thickness of the internal anal sphincter was significantly correlated to resting pressure(r=−0.67,P=0.0001).CONCLUSION:The ratio between the thickness of the external and internal sphincter muscles measured on the sonography screen was significantly reduced in patients with neurogenic incontinence compared with controls(P<0.01).