The motor unit fibre density in the external anal sphincter muscle has been measured in 13 normal subjects aged 18 to 78 years, and in 14 patients with ano-rectal incontinence, aged 17 to 72 years. The mean fibre density in the normal subjects was 1.5 +/- 0.16, and in the incontinent subjects it was 2.17 +/- 0.28 (p less than 0.001). These findings provide support for the suggestion that ano-rectal incontinence commonly has a neurogenic basis.