Purpose. To determine the rates of sildenafil (Viagra) use among different populations, primarily among men who have sex with men, and to measure the association of sildenafil use with increased sexual risk behavior and sexually transmitted diseases (STDs), including human immunodeficiency virus (HIV) infection. Methods. The National Center for Biotechnology Information PubMed was searched using a variety of terms for relevant publications from January 1999 to July 2004. In addition, all scientific abstracts from national and international conferences on STDs from January 1999 to July 2004 were searched. Relevant journal articles and scientific abstracts presenting original data and meeting given criteria were included. Results. Fourteen studies met the inclusion criteria. Seven of the 11 studies in men who have sex with men showed sildenafil use rates >10% (range 3%32%). Increased odds of unprotected anal sex with a partner of unknown or serodiscordant HIV status ranged from 2.0 to 5.7 times (mean = 3.9) for sildenafil users versus nonusers. The risk of sildenafil use and STD diagnosis among HIV-positive men who have sex with men was 1.92 (P = 0.05), and the odds of sildenafil use among those newly HIV infected was 2.5 (95% CI 1.14.1). Conclusions. Most studies reported frequent sildenafil use in men who have sex with men, and several showed independent associations between use of the drug and sexual risk behavior, as well as an increased risk for STDs, including incident HIV infection. Although future research among more varied participants is needed, these results warrant a multi-faceted response to reduce the misuse of sildenafil and its consequences, particularly among men who have sex with men. (Author' s abstract)