Infection with human papillomavirus (HPV) serotypes 16 and 18 in men who have sex with men is a cause of considerable morbidity associated with anal intraepithelial neoplasia. This study shows that the HPV vaccine decreases the risk of HPV-associated anal disease. Anal cancer is biologically similar to cervical cancer, including having a causal relationship with human papillomavirus (HPV) infection. 1 Although HPV type 6 (HPV-6) or HPV type 11 (HPV-11) alone is rarely causal, the proportion of anal cancers associated with infection with HPV type 16 (HPV-16) or HPV type 18 (HPV-18) is as high as or higher than the proportion of cervical cancers. 1 Just as cervical cancer is preceded by high-grade cervical intraepithelial neoplasia (grade ≥2), anal cancer is preceded by high-grade anal intraepithelial neoplasia (grade 2 or 3). 2 – 4 Although not yet formally demonstrated, prevention or treatment of high-grade anal . . .