The external anal sphincter (EAS) anatomy is complex, and no exact technique of needle electrode insertion into it for electromyography (EMG) has been described. To define optimal positions for needle electrode insertions, EAS muscle topography was studied by concentric needle EMG. Fifteen women without uroneurological disorders were examined. Perpendicular insertions were made superficially (just under the mucosa) at the mucocutaneous junction, 5 and 10 mm more proximally (toward the anus), and at the anal orifice. In addition, at the anal orifice, deeper insertions were made. Superficially, EMG activity was detected at the mucocutaneous junction in 9 (60%) subjects. In the remaining 6, the muscle was found either 5 mm (in 5) or 10 mm (in 1) more centrally. At the anal orifice, superficial EMG activity was present in 67% of women. On deep insertion (15-25 mm) at the anal orifice, muscle was always present. It is suggested that, in further studies, the portions of the EAS muscle examined should be specified.