Among 280 infants under 1 year of age with congenital heart disease autopsied at the University of Colorado Health Sciences Center between 1959 and 1978, there were six instances of grade IV1 pulmonary artery hypertension. Five were patients with ventricular septal defect (four associated with other cardiovascular malformations). The sixth was a patient with atrioventricular canal. The youngest was 2 1/2 months of age. Advanced degrees of pulmonary hypertensive arteriopathy (grade IV or more) have been said to be rare in infants, especially under the age of 1 year. The fact that all of these cases occurred within the last few years of the study suggests the possibility of improved supportive care leading to the prolonged survival of infants who might otherwise have died prior to developing severe disease. In addition, the role of altitude in accelerating the arteriopathy must be considered in the present series. In any case, this unexpected increase in the frequency of severe pulmonary hypertensive arteriopathy should stimulate consideration of early surgical correction of the underlying cardiovascular malformation, especially in areas of relatively high altitude.