To determine the microbial cause and the clinical and pathologic correlates of anorectal and intestinal symptoms in homosexually active men, we performed comprehensive microbiologic studies, anoscopy, sigmoid-oscopy, and rectal biopsy in men examined in a clinic for sexually transmitted diseases. Enteric pathogens were found in 95 of 119 consecutive homosexual men with anorectal or intestinal symptoms and in 29 of 75 randomly selected homosexual men without such symptoms (P less than 0.001). The syndromes of proctitis, proctocolitis, and enteritis were differentiated on the basis of predominant symptoms and findings on anoscopy and sigmoidoscopy. Neisseria gonorrhoeae, herpes simplex virus, Chlamydia trachomatis (non-lymphogranuloma venereum serotypes), and Treponema pallidum were associated with 80 per cent of cases with symptomatic proctitis. Known causes of colitis, including Campylobacter jejuni, Campylobacter fetus fetus, Shigella flexneri, Chl. trachomatis (lymphogranuloma venereum serotypes), Entamoeba histolytica, and Clostridium difficile, were identified in 60 per cent of the cases of proctocolitis. Giardia lamblia was the only agent significantly correlated with enteritis. These data demonstrate that intestinal symptoms in homosexual men are attributable to a complex spectrum of microorganisms, but that careful clinical classification can serve as a guide to the selection of microbiologic studies and to a rational initial choice of therapy.