Two patients with extensive warts which were stable for two years or more were treated with human interferon-alpha to assess the ability of interferon to affect this benign tumor of viral etiology. Intramuscular administration of 96.6 and 135 million units over 12-15 weeks produced softening and decreased scaling of each patient's warts. Double blind, placebo-controlled intralesional injections resulted in progressive disappearance of interferon treated warts. A dose response relationship was shown in eight warts. The minimum effective dose was 1.2 x 10(6) units injected over 15.5 weeks.