The aims of the study were to determine what microscopic changes occur in the rectal mucosa of men who have had anal intercourse and to correlate the sigmoidoscopic and microscopic appearances. Histological abnormalities were found in 29 of 100 men who attended consecutively a sexually-transmitted diseases clinic. The histopathology of rectal gonorrhoea, as observed in 18 patients, is described as are the microscopic findings in the rectal mucosa of 10 patients with early syphilis. Of 70 men without any detectable rectal infection, biopsies from 15 (21.4%) were abnormal. Intestinal spirochaetosis was observed in biopsies from 36 of these 100 men. With the use of strict criteria to describe the macroscopic appearance of the rectal mucosa, the sigmoidoscopic findings correlated well with the histology.