The arterial oxygen tensions of 70 infants receiving oxygen therapy were correlated with the appearance of their retinal vessels. Eighteen of the infants (25.7%) showed retinal vessel changes when arterial oxygen tension (Pao2) was between 100 and 400 mm Hg. Their arterial carbon dioxide tension (Paco2) and hydrogen ion concentrations ([H+]) at the peak Pao2 level were also analyzed and found to be not obviously related to the changes. The incidence of retinal vessel changes increased with decreasing birth weight, supporting previous observations concerning the inverse relationship between the incidence of retinal damage resulting from oxygen toxicity and the degree of immaturity of the infant.