Condylomata acuminata (CA) caused by human papillomavirus (HPV) is a common sexually transmitted disease with half a million new cases diagnosed in the United States per year. Recurrence is a major challenge for CA treatment. The investigators have demonstrated that FOXP3+ regulatory T (Treg) cells mediate the immunosuppression in large genital warts. And low-dose cyclophosphamide (CY), a conventional chemotherapy drug, has been reported to selectively deplete Treg cells in cancer patients. Therefore, the investigators hypothesized that low-dose CY can be used to treat genital warts. In this study, 104 CA patients have been recruited for clinical trial with a 1:2 randomization. Among them, 64 patients received low-dose cyclophosphamide and 32 received placebo. In 8 extra patients, high-dose cyclophosphamide was given.