Full thickness specimens of normal colon (n = 15), and colon from patients with diverticular disease (n = 5) were obtained at operation or autopsy. In the isolated submucosa the ultrastructure of the constituent collagen fibres was examined by transmission electron microscopy. Collagen fibrils in the left colon become smaller (p less than 0.001) and more tightly packed (p less than 0.001) than those in the right colon with increasing age. This difference is accentuated in diverticular disease (p less than 0.01). Factors which contribute to the development of colonic diverticulosis, such as raised intraluminal pressure, may be responsible for premature change in submucosal structure.