Fifteen patients with chronic diarrhea and fecal incontinence were admitted to a clinical research center and treated for 3 days with either placebo or diphenoxylate with atropine (Lomotil). The patients were then crossed over to the alternate medication. Lomotil had no effect on rectal or anal sphincter pressure or on continence for saline that had been infused into the rectum. However, Lomotil therapy reduced average stool frequency (from 4.9 to 2.6 times/day) and average stool weight (from 460 to 256 g/day). These results suggest that temporary or intermittent therapy with Lomotil and related drugs might benefit patients with chronic diarrhea and fecal incontinence. They should do this by virtue of a reduction in stool frequency and stool volume, without a deleterious effect on the defense mechanisms against incontinence.