Type 1 fibre predominance was found in the external anal sphincter, puborectalis and levator ani muscles of 17 control subjects, and of 16 patients with ano-rectal incontinence. In the external anal sphincter and puborectalis muscles of the control subjects the mean diameter of Type 2 fibres was slightly greater than that of Type 1 fibres, but in the levator ani muscles of control female subjects the mean diameter of Type 1 fibres was much greater than that of Type 2 fibres. In the patients with anorectal incontinence there was marked hypertrophy of fibres of both histochemical types. This was most marked in the puborectalis and external anal sphincter muscles. In 12 of the 16 incontinent patients there were histological and statistical features consistent with a neurogenic disorder. These histometric studies provide a quantitative basis for physiological and pathological studies of these muscles in incontinence and other anorectal disorders.