Imiquimod, an imidazoquinoline amine, is an immune response modifier recently approved for the topical treatment of external genital and perianal warts. Although the majority of immunomodulatory agents available or in development inhibit pathways involved in immune activation, imiquimod is unique in that it activates immune function. The exact mechanism of imiquimod’s antiviral activity is unknown; however, its effects are likely to be related to its immunomodulating properties. Although in vitro studies have shown that imiquimod has no direct antiviral effects, the drug does exhibit antiviral and antitumor effects in vivo through induction of cytokines and enhancement of cell-mediated cytolytic antiviral activity. Imiquimod stimulates the innate immune response through induction of cytokines, and the cellular arm of acquired immunity through induction of interferon-⍺ (IFN-⍺), IFN-γ, and interleukin-12. Results from animal studies have indicated a possible use for imiquimod in the prevention and treatment of herpes simplex virus infection. In addition, recent studies demonstrated that imiquimod activates Langerhans’ cells and enhances allergic contact hypersensitivity. (J Am Acad Dermatol 2000;43:S6-11.)