BACKGROUND Genital warts are a common sexually transmitted disease caused by human papillomavirus. Imiquimod is a novel immune-response modifier capable of inducing a variety of cytokines, including interferon alfa, tumor necrosis factor-alpha, as well as interleukins 1, 6, and 8. In animal models imiquimod has demonstrated antiviral, antitumor, and adjuvant activity. In vitro, imiquimod has no antiviral or antitumor activity. OBJECTIVE Our purpose was to determine the safety and efficacy of topical imiquimod for the treatment of external genital warts. METHODS This prospective double-blind, placebo-controlled, parallel design clinical trial was performed in three outpatient centers, a public health clinic, a university-based clinic, and a private practice. One hundred eight patients with external genital warts (predominantly white men) were entered into the trial. Fifty-one patients were randomly selected to receive 5% imiquimod cream; 57 patients were randomly chosen to receive placebo cream. Study medication was applied three times weekly for up to 8 weeks. Patients whose warts cleared completely were observed for up to 10 weeks to determine recurrence rates. RESULTS In the intent-to-treat analysis, the warts of 37% (19 of 51) of the imiquimod-treated patients and 0% (0 of 57) of the placebo group cleared completely (p < 0.001). In addition, many patients experienced a partial response. A reduction in baseline wart area of 80% or more was observed in 62% of imiquimod-treated patients (28 of 45) and 4% of the placebo group (2 of 50) (p < 0.001); a 50% reduction or more in wart area was noted in 76% of imiquimod-treated patients (34 of 45) and 8% of placebo recipients (4 of 50) (p < 0.001). Of imiquimod-treated patients whose warts cleared completely and who finished the 10-week follow-up period, 19% (3 of 16) experienced recurrences of warts. Imiquimod-treated patients experienced a significantly greater number of local inflammatory reactions than the placebo group. Symptoms and signs associated with the local inflammatory reactions included itching (54.2%), erythema (33.3%), burning (31.3%), irritation (16.7%), tenderness (12.5%), ulceration (10.4%), erosion (10.4%), and pain (8.3%). There were no differences in systemic reactions or laboratory abnormalities between treatment groups. CONCLUSION Topical 5% imiquimod cream appears to have a significant therapeutic effect in the treatment of external genital warts.