Patients with condyloma acuminatum were treated with subcutaneous injections of interferon alpha three times weekly for three weeks. Natural killer cell activity pre-treatment was the same for non-responders to interferon therapy as for responders. After two weeks of therapy, however, the natural killer cell activity of responders was significantly greater than that for non-responders. This difference in activity was also observed four weeks following therapy. It therefore appears that measurement of natural killer cell activity has value in predicting which patients with condyloma acuminatum will experience good clinical response to interferon therapy.