The potential role of sildenafil (Viagra) in the risk of HIV and sexually transmitted disease (STD) transmission was evaluated among gay or bisexual men seeking public STD services in San Francisco. Viagra users reported greater numbers of recent sex partners, higher levels of unprotected anal sex with an HIV-positive partner, and higher rates of prevalent STD than non-users. Moreover, mixing Viagra with illicit drugs was commonly reported. Further studies are needed to determine whether a causal role exists.