The cause of colonic diverticula is myostatic spasm. A low-residue diet contributes to this state as transit time through the bowel is slowed. Information gathered in 102 cases was analysed. The smooth muscle architecture of specially prepared operative specimens of diverticulosis were examined. In every case an asymmetric contracture of the smooth musculature led to the formation of gaps. Here the mucosa and muscularis mucosae can protrude through the bowel wall. The muscular spasm is responsible for incarceration of the diverticula. Inflammatory diverticulitis is compared to appendicitis.