A series of 128 biopsy specimens from anal condylomas in 73 homosexual or bisexual and 38 heterosexual men (mean (SD) age 31.8 (9.6) years) were subjected to histological assessment and human papillomavirus (HPV) typing by in situ DNA hybridisation with 35S-labelled HPV 6, 11, 16, 18, 31, and 33 probes. Most patients were also tested serologically for antibodies to human immunodeficiency virus (HIV). As evaluated on light microscopy, most (74%, 95/128) of the lesions were exophytic (papillary) acuminate warts, 15% (19) were flat, and 11% (14) were pigmented papulous lesions. No signs of anal intraepithelial neoplasia (AIN) were seen in 70% (90) of the 128 biopsy specimens (NAIN), 27% (35) were classified as showing AIN I, and another 2% (three) as AIN II. AIN was significantly (p less than 0.05) more often associated with papulous lesions, only 43% (6/14) of which showed NAIN compared with 72% (68/98) of acuminate condylomas. The duration of disease was directly related to the presence and severity of AIN in the lesions; thus in 47 lesions that had been present for more than 12 months, NAIN was found in 31 (66%), AIN I in 14 (30%), and AIN II in two (4%). HPV DNA of at least one of the six types tested for was detected in 109/125 (87%) lesions. HPV 6 and HPV 11 were the two most common types, comprising 57% (62) and 37% (40), respectively, of the 109 HPV DNA positive cases. Only seven (6%) biopsy specimens were associated with any of HPV types 16, 18, 31, or 33, which carry a high risk of potential malignant transformation. No association was found between sexual preferences of patients and the incidence of any of the various HPV types. Neither did the distribution of the various HPV types differ between men with antibody to HIV and those without antibody. All the men with antibody to HIV were homosexual or bisexual. On microscopy, 93% (38) of 41 lesions containing HPV 11 and 75% (48/64) of HPV 6 lesions were of the acuminate wart type; in comparison, the remaining 16 HPV 6 lesions were equally either flat or papulous (eight, 13% each). Of the 64 HPV 6 and 41 HPV 11 associated lesions, 73% (47) and 63% (26), respectively, were classified as NAIN. Only two lesions were associated with HPV 16, and both showed mild dysplasia. On the other hand, two HPV 6 induced lesions were associated with AIN II. No differences were found between HPV 6 and HPV 11 in duration of disease; (39%, and 27% respectively, had been present for more than 12 months). The results showed that overt anal wart disease was associated with HPV types 6 and 11 in most cases. Although HPV types considered as being of higher oncogenic potential were detected relatively rarely, the associated AIN in a relatively high proportion (31% 32/105) of HPV 6 or 11 induced lesions indicated that a malignant potential, even for HPV 11 associated anal warts, cannot be excluded.