AbstractThe motor unit tibre density in the external anal sphincter has been measured in 25 female subjects with genuine stress incontinence and in 14 normal female subjects. The mean fibre density in the normal subjects was 1.5 ± 1.5 and in the incontinent patients 1.95 ± 0.32 (p < 0.002). These findings show objective evidence of denervation of the muscle and suggest that genuine stress incontinence often has a neurogenic aetiology.