Forty-nine subjects were enrolled in a study comparing two dosages of parenterally administered interferon (IFN)-β in combination with cryotherapy for the treatment of anogenital warts. Subjects were randomized to receive subcutaneous injections of either 2 x 106 or 4 x 106 IU/m2 of IFN-β (Biogen) three times a week for a total of 6 weeks. Cryotherapy was administered concomitantly by aerosolization of liquid nitrogen at 10-day intervals. Systemic side- effects were modest in intensity and included fever, chills, myalgia, and headaches (flu-like symptoms). During the first 2 weeks of therapy, they were more common in the high dose group than in the low dose group (P = 0.02). Using survival analysis, there was no significant difference between the two groups in rates of resolution of warts present at baseline (P = 0.62). However, the rate of new lesion formation during the study was significantly lower in the high dose group (P = 0.04).