This study aimed to examine the prevalence of anal cytological abnormalities in groups of HIV-infected and non-infected homosexual men, and to monitor changes with time. Dyskaryosis suggestive of anal intraepithelial neoplasia (AIN) was noted in 24 (30%) of the 80 satisfactory anal smears from 66 HIV-seropositive homosexual men; such changes were found in only 7 (4.7%) of the 149 satisfactory smears from 181 HIV-seronegative homosexual men (P < 0.005), and in none of 34 satisfactory preparations from 51 HIV-seronegative heterosexual men. In the follow-up of 20 HIV-seropositive men, the severity of the cytological abnormalities found in 2 men increased, with the most recent smear showing changes suggestive of AIN III; one of these men subsequently developed anal cancer. Smears from 4 men showed apparent regression in the degree of dyskaryosis. Although the numbers of patients studied were small, there appeared to be a trend towards a more severe degree of dyskaryosis in those men with increasing immunodeficiency. There was no significant difference in the detection of human papillomavirus types 6b, 11, 16 and 18 between HIV-infected and non-infected men.