Five infants with symptomatic cytomegalovirus infections and high urinary titers of cytomegalovirus were treated with interferon in doses of 1.7-3.5 X 10(5) reference units/kg per day for seven to 14 days. Six courses of treatment were given. One of two infants treated with the largest dose of interferon had transient suppression of viruria; no suppression was noted during the other five courses of treatment. Adverse effects noted included a low rate of weight gain, a transient elevation of serum aspartate aminotransferase and fever. Further trials are needed to determine whether larger doses of a more purified preparation of interferon will eliminate viruria in infants with sytomegalovirus infection, but careful assessment of toxicity will be necessary in this population of patients.