A 27-month-old boy developed fatal measles pneumonia during the course of severe hemorrhagic varicella with pulmonary involvement. His humoral antibody response to both viruses was normal. Neither historical nor histologic evidence suggests any defect of cellular immunity. It is hypothesized that interferon, produced during the first (varicella) infection, was not effective in protecting against the second (measles) virus because of its limited location in relation to the timing and progression of the latter illness. It is probable that the ventilation-perfusion deficit, resulting from the combined effect of the two viral infections, was severe enough to cause death.