Sixteen cases of the megasigmoid syndrome were found. This uncommon disorder is predominantly observed in neuropsychiatric hospitals but not exclusively in psychotic patients. It is confined to the rectosigmoid. Significant features are paucity of symptoms despite severe constipation and bowel distention, relaxed anal sphincter with fecal incontinence, chronic proctosigmoiditis with fecal concrements and stercoraceous ulcers, and preservation of intramural ganglion cells. Roentgenographically, three different patterns can be distinguished. Presence of rectal gas with the patient in prone position rules out a sigmoid volvulus. Serious complications are perforation of stercoraceous ulcers, chronic toxemia, intestinal obstruction, and volvulus. The mortality can be lowered by early detection, especially from chest roentgenograms, and by appropriate therapy. A correlation appears to exist between the megasigmoid syndrome and lesions of the brain and the autonomic nervous system.