Sixty-six patients with recurrent respiratory papillomatosis of juvenile onset were treated for six months with interferon alfa-n1 (Wellferon) in a randomized crossover trial. Half received interferon alfa-n1 intramuscularly at a dosage of 5 megaunits per square meter daily for 28 days and then thrice weekly for five months, followed by six months of observation. The other half were observed for six months and then treated. Operations were performed every two months to assess disease extent by a scale developed for this purpose. The score for the patients during the first observation period was stable. There was a statistically significant lowering of score in patients receiving interferon alfa-n1 during both periods of drug administration. Eight of 57 patients with assessable airway disease achieved complete remission, as did one additional patient with disease limited to the nasopharynx. No patients achieved complete remission during six months of observation alone. This difference was statistically significant. Patients without tracheostomy were significantly more likely to achieve remission than those with a tracheostomy. The patients who were observed after discontinuation of the drug therapy showed a significant rise in score within four months. Symptoms of toxicity included transient fever, fatigue, nausea, and headache. Elevations in serum aspartate aminotransferase levels occurred in 64% of the patients. There was an inverse correlation between age and the ability to tolerate the medication. The dose studied may be close to the maximum tolerated dose. It appears that interferon alfa-n1 as an adjuvant to routine surgical management is effective in slowing the growth of respiratory papillomas.