To assess the efficacy of a novel glycosylated mammalian cell derived recombinant human interferon-beta (r-hIFN-beta-1a) in the intralesional treatment of genital condylomata acuminata. The study was randomized, double-blind and placebo-controlled. Patients (n=60) with up to 8 distinct condylomata acuminata were randomized to receive either one million international units (IU) of r-hIFN-beta-1a or placebo intralesionally into each lesion, 3 times a week, for a total of 9 occasions. Biopsies were taken from each patient before enrolment to allow human papillomavirus (HPV) testing, and patients were tested for the development of anti-IFN-beta antibodies. Efficacy was assessed by measuring the complete response rate 3 months after treatment. The complete response rate was not significantly better with r-hIFN-beta-1a than with placebo. However, after 3 months, 73.3% of patients treated with r-hIFN-beta-1a had experienced at least a partial response to treatment, compared with 33.3% of placebo-treated patients. At 19 days and 6 weeks, r-hIFN-beta-1a produced a significantly larger reduction in the area of condylomata. Lesions with detectable HPV6 or 11 showed a trend towards a better response rate to treatment with r-hIFN-beta-1a than lesions where no HPV DNA was detected. The treatment was well tolerated. In the 5 patients who developed non-neutralizing anti-IFN-beta antibodies, therapeutic efficacy was not compromised. Intralesional r-hIFN-beta-1a was effective in the reduction of the size of genital condylomata acuminata.