Human leukocyte interferon cream was evaluated in a double blind placebo-controlled trial in 13 patients with widespread vaginal flat condylomatous dysplasia. Patients applied 12 million units of interferon vaginal cream daily into vagina during four two-week treatment courses separated by one-week intervals. Five of eight patients treated with interferon showed clear remissions in colposcopy. Cytologic examinations, however, revealed cells typical of condyloma in all cases. Two responding patients relapsed one and two months after the treatment. Among five patients using placebo, the lesions remained unchanged in three patients and progressed in two. Interferon treatment did not alter the microbiologic or cytologic picture of Papanicolaou smears, but lymphocytic infiltrations in the stroma next to condylomas were seen in histologic samples. No overt side effects were observed during interferon treatment.