Cells from the anorectal mucosa of 61 homosexual men were examined microscopically for evidence of papillomavirus infection and dysplastic changes. There was cytological evidence of dysplasia with concomitant features of human papillomavirus (HPV) infection on at least one occasion in 24 men and of papillomavirus infection without dysplasia on at least one occasion in a further 26: dysplasia was present for over one year in 9 of 14 men who were re-examined. Dysplasia was associated with a history of anal warts, frequent receptive anal intercourse, presence of serum antibody to human immunodeficiency virus (HIV), and immune dysfunction as judged by a low CD4/CD8 ratio, but not with the lifetime number of sexual partners. The association of longlasting dysplasia with anti-HIV was independent of the association with immune dysfunction. Thus infection of anorectal mucosal cells with papillomavirus seems to be frequent among homosexual men and may predispose to dysplasia.