We investigated retrospectively whether the preexistence of inflammation-producing illnesses such as viral respiratory tract infections contributed to the development of high-altitude pulmonary edema in children. We found that the large majority of native low-altitude children, but not adults, who had this form of edema after traveling to high altitude also had evidence of a preexisting illness. We speculate that the release of inflammatory mediators associated with these illnesses may be tolerated at sea level but may predispose children to increased capillary permeability when superimposed on hypoxia and, possibly, cold and exercise.