Imiquimod, the first member of a new class of immune response modifiers, is approved for the treatment of external genital and perianal warts. The clinical effect of imiquimod stems from cytokine-induced activation of the immune system. Topical application of imiquimod elevates the production of cytokines, including the principal cytokine for antiviral activity, interferon-alpha. This is the initial event in an immunological cascade resulting in the stimulation of the innate immune response as well as the cell-mediated pathway of acquired immunity. This immune modification mediates the indirect antiviral, antiproliferative and antitumor activity of imiquimod in vivo. These properties highlight the potential of imiquimod not only as an effective treatment for genital warts, but also as a treatment for other cutaneous viral infections and cutaneous neoplasms.