BACKGROUND: Cimetidine, an H2-receptor antagonist, has been used successfully to treat patients with mucocutaneous candidiasis, common variable immunodeficiency, herpes simplex, and herpes zoster because of its immunomodulatory effects. Recently, some trials have suggested that cimetidine may also be useful for the treatment of warts. OBJECTIVE: The aim of the present study was to determine whether cimetidine is effective in the treatment of warts. METHODS: Seventy patients with multiple warts were included in a placebo-controlled, double-blind study. Patients were randomly allocated to treatment groups equally. The groups received cimetidine, 25 to 40 mg/kg daily, or placebo for 3 months. Patients were examined at monthly intervals. RESULTS: At the end of the therapy, 28 cimetidine-treated and 26 placebo-treated patients were examined to determine the efficacy of treatment. Cure rates obtained were 32% (9 of 28) in the cimetidine-treated group and 30.7% (8 of 26) in the placebo-treated group. No significant difference was found between cimetidine and placebo in effectiveness (p = 0.85). CONCLUSION: Our results show that cimetidine is no more effective than placebo in the treatment of patients with common warts.