During multiport manometric recordings of the anorectum, 35 of 140 patients with idiopathic fecal incontinence had abnormally weak sphincters that did not relax during rectal distention, suggesting impaired function of the internal anal sphincter. This study documents this finding and compares anorectal function in these 35 patients with 25 matched normal subjects and the remaining 105 incontinent patients. Basal and squeeze pressures were significantly lower in the incontinent study group than in the other two groups. During rectal distention, 25 of 35 patients showed no anal relaxation at any volume, whereas 10 patients showed some relaxation after 60-ml distention because of the postinflation rebound in internal anal sphincter tone. The electrical responses of the external anal sphincter to rectal distention and to increases in intraabdominal pressure were greater in patients than in normal subjects, but the anal pressures were lower and anal leakage was common. Rectal sensations were not significantly different from normal subjects. These results indicate that 25% of patients with idiopathic fecal incontinence have markedly impaired function of the internal anal sphincter, but these patients also have extremely weak external sphincters.