SummaryThe dissecting‐microscopic appearance of the human rectal mucosa was studied in 147 biopsy specimens. The normal mucosa is divided into mucosal units by a “honeycomb” pattern of the rectal microvasculature, the center of each unit containing a refractile “pit” which may represent the mouth of the rectal crypt. The changes occurring in ulcerative colitis were graded according to severity of disturbance of the vascular channels, ranging from dilatation, neoformation and tortuosity of the individual vessels to complete disorganization of the “honeycomb” pattern. The vascular and “pit” changes in ulcerative colitis and other diseases of the rectum are described. The pathogenesis of the dissecting‐microscopic changes is discussed in relation to the normal vascular pattern of the human colonic mucosa.