We conducted a multicenter double-blind study comparing human recombinant intralesional alpha-2 interferon (IFN) and placebo in 237 patients with the clinical diagnosis of condyloma acuminatum or verruca plantaris. A single wart on each patient was injected with 0.1 mL containing 10(6) IU of IFN, 10(5) IU of IFN, or placebo three times weekly for three weeks, and the response to treatment was followed up for 12 weeks. Among 91 of the 114 patients with condyloma acuminatum who completed the study, complete clearing of the treated wart occurred in 16 (53%) of 30 patients receiving 10(6) IU of IFN compared with six (19%) of 32 receiving 10(5) IU of IFN and four (14%) of 29 receiving placebo. In the group of 100 patients with plantar warts, there was no apparent benefit associated with interferon administration. Seven patients (3%) had treatment discontinued due to adverse reactions. Intralesional alpha-2 IFN is of benefit in the treatment of a single condyloma; its role in the treatment of multiple lesions remains to be clarified. Its role in the treatment of verruca plantaris, where no response was seen, also remains to be clarified.