&NA;Electron microscopy of the colonic mucosa was performed in 33 patients with acute ulcerative colitis. The process of the development of crypt abscesses was studied. Inflammatory cells migrated intercellularly from the lamina propria and did not destruct the neighboring epithelial cells. Abnormal maturation of the epithelial cells was observed. Loose connection and separation of immature epithelial cells may promote migration of inflammatory cells.