Compared to the general population the incidence of anal intraepithelial neoplasia (AIN) and anal carcinoma (AC) amongst men who have sex with men (MSM) is extremely high (above 70/100,000). While many opportunistic infections have declined since the introduction of highly active antiretroviral therapy (HAART), the incidence of AC has not fallen. In contrast, the HAART-related improvement of survival seems to result in an increased risk of AC in HIV-infected MSM. Screening for cervical intraepithelial neoplasia (CIN) with cervical cytology and early treatment has resulted in a significant decline in the incidence of cervical carcinoma. Like cervical cancer, AC may be preventable through identification and treatment of its precursors. Nevertheless, there has never been an effort to implement an anal cytology screening program for HIV-infected MSM. Persistent cervical infection with high-risk HPV-types is indicative for the development of CIN and cervical cancer. As the prevalence of genital HPV-infections in HIV-infected women and men is very high (up to > 90%), the predictive value of qualitative HPV-DNA detection is limited for cervical cancer or AC prevention. However, several studies have shown that the number of HPV-DNA copies in cervical scrapes may be predictive of the severity of underlying cervical dysplasia. Thus, this prospective study mainly focuses on the predictive value of HPV-DNA load for the development and clinical progression of AIN in HIV-infected MSM. Besides, HPV-E6/E7-oncogen-expression using RT-PCR will be determined. Moreover, the course of HPV viral load under therapy for anal intraepithelial neoplasia, e.g. topical treatment with imiquimod, will be evaluated. Additionally, immunohistochemical determination of several proliferative biomarkers as well as cytokines will be performed.