We compared the potential value of rectal-screening cultures for Chlamydia trachomatis and Neisseria gonorrhoeae among homosexual men attending a sexually transmitted disease clinic and also assessed the relation of either infection to patient age, symptoms, history of previous infection, and likelihood of early treatment. Of 1,429 homosexual men cultured over a six-month period, 118 (8%) had gonococcal rectal infection, 72 (5%) had chlamydial rectal infection, and 15 (1%) had both. The prevalence of rectal chlamydial infection was strongly related to age, with the highest occurrence in adolescents. C. trachomatis was isolated more often from men infected with N. gonorrhoeae (11%) than from men not infected with N. gonorrhoeae (5%; P = .01), and simultaneous urethral infection in patients with positive rectal cultures occurred more frequently in men infected with N. gonorrhoeae than in men infected with C. trachomatis (27% vs. 14%, respectively; P = .05). The majority of rectal infections with N. gonorrhoeae and C. trachomatis in this population were asymptomatic. Although 80% of the patients who had rectal infection with N. gonorrhoeae were appropriately treated on the initial clinic visit, only 18% of those with rectal chlamydial infections received proper treatment (P less than .0001). The yield of patients who were newly identified and brought to treatment was 4.8 patients per 100 screening cultures for C. trachomatis vs. 1.9 patients per 100 screening cultures for N. gonorrhoeae.