We studied 16 patients with diabetes and fecal incontinence. The onset of incontinence coincided with the onset of chronic diarrhea in most patients. Episodes of incontinence occurred when stools were frequent and loose; however, 24-hour stool weights were usually within normal limits. All patients had evidence of autonomic neuropathy, and one third had steatorrhea. Incontinent diabetics had a lower mean basal anal-sphincter pressure than 35 normal subjects (63 +/- 4 vs. 37 +/- 4 mm Hg; P less than 0.001), reflecting abnormal internal-anal-sphincter function. The increment in sphincter pressure with voluntary contraction (external-sphincter function) was not significantly different from normal. Incontinent diabetics also had impaired continence for a solid sphere and for rectally infused saline. In contrast, 14 diabetics without diarrhea or incontinence had normal sphincter pressures and normal results on tests of continence, even though 79 per cent had evidence of autonomic neuropathy and nearly half had steatorrhea. We conclude that incontinence in diabetic patients is related to abnormal internal-anal-sphincter function, and that as a group, diabetics without diarrhea do not have latent defects in continence.