We measured anorectal sensory and motor function in 11 patients with multiple sclerosis and fecal incontinence, 11 continent patients with multiple sclerosis, 10 diabetics with fecal incontinence, and 12 healthy control subjects. The threshold volume at which patients with multiple sclerosis and fecal incontinence experienced rectal sensation was higher than that in healthy controls (42.7 +/- 6.2 mL vs. 13.3 +/- 2.8 mL; P less than 0.01) and was similar to that in incontinent diabetics (36.5 +/- 5.7 mL). Patients with multiple sclerosis and incontinent diabetics also showed increased thresholds of phasic external sphincter contraction compared with controls (P less than 0.05). Diabetics with incontinence had reduced resting and maximal voluntary anal sphincter pressures compared with controls (P less than 0.05), whereas patients with multiple sclerosis and incontinence showed only decreased maximal voluntary anal sphincter pressures (P less than 0.01 vs. controls and diabetics). Incontinent patients with multiple sclerosis also required smaller volumes of rectal distention to inhibit internal sphincter tone compared with diabetics and controls (P less than 0.01). Decreased maximal voluntary squeeze pressures were less severe in continent patients with multiple sclerosis than in incontinent patients with multiple sclerosis. We conclude that impaired function of the external anal sphincter and decreased volumes of rectal distention to inhibit the internal anal sphincter or both may contribute to fecal incontinence in multiple sclerosis. In addition, increased thresholds of conscious rectal sensation in some incontinent patients with multiple sclerosis and diabetes mellitus may contribute to fecal incontinence by impairing the recognition of impending defecation.