A rising incidence of syphilis and lymphogranuloma venereum (LGV) underscores the importance of recognizing these sexually transmitted infections (STI) in routine anocolonic biopsies. To increase awareness of their morphologic manifestations, we undertook a clinicopathologic study of our experience: syphilis (7 patients, 7 specimens), LGV (2 patients, 4 specimens), and syphilis/LGV (1 patient, 3 specimens). The diagnoses of all study specimens were confirmed with pertinent clinical studies. All study patients were human immunodeficiency virus positive, and all 9 with available history were men who have sex with men. The majority presented with bleeding (9), pain (6), and tenesmus (4). Ulcerations were the most common endoscopic abnormality (7), whereas mass lesions were confined to the syphilis group (4). None of the initial impressions included LGV, and syphilis was prospectively suggested only by pathologists (6 of 8) without the knowledge of clinical information and on the basis of morphology. Alternative impressions included condyloma acuminatum (3), inflammatory bowel disease (3), and malignancy (2), among others. All study specimens shared the following histologic core features: an intense lymphohistiocytic infiltrate with prominent plasma cells and lymphoid aggregates, only mild to moderate acute inflammation, minimal basal plasmacytosis and crypt distortion, and only rare granulomas and Paneth cell metaplasia. The spirochetes were focally demonstrated on a Treponema pallidum immunohistochemical stain (1) but not on silver stains (3). All patients with available follow-up data showed resolution of symptoms and imaging abnormalities after STI therapy (6). In summary, we report a unique pattern of STI proctocolitis consistently identified in patients with serologically confirmed syphilis and/or LGV infection; pertinent STI therapy leads to resolution of clinical abnormalities. This histologic pattern is important to recognize for timely treatment, for prevention of onward STI transmission, and to avoid the diagnostic pitfalls of inflammatory bowel disease or malignancy.