One hundreds seventy-one homosexual men, 96 of whom had symptoms suggestive of proctitis and 75 of whom had no such symptoms, were consecutively enrolled in a study of the prevalence, clinical spectrum, and histopathology of Chlamydia trachomatis rectal infections. C. trachomatis was isolated from the rectums of 14 men. Three of the isolates, which had lymphogranuloma venereum (LGV) immunotypes, were recovered from three men with symptoms and signs of severe proctitis, two of whom had granulomatous inflammation that was observed at rectal biopsy and was initially suggestive of Crohn's disease. Eleven isolates, which had non-LGV immunotypes, were obtained from eight symptomatic and three asymptomatic men, all of whom had fecal leukocytes and mild abnormalities of the mucosa present at sigmoidoscopy, usually with mild nongranulomatous inflammatory changes that were seen on rectal biopsy. These observations suggest that the presence of LGV immunotypes of C. trachomatis in the rectum is associated with severe acute proctitis that mimics Crohn's disease of the rectum, whereas the non-LGV immunotypes are associated with a mild proctitis with or without symptoms.