Refractive errors were measured in 514 healthy subjects, aged 1, 1.5, 2.5, 4, 6, 9, 12, 18, 24, 30, 36, and 48 months. Three hundred of the subjects were aged 12 months or younger. Age at refraction varied from the nominal age by no more than 10 days in the first year, and 14 days thereafter. The median number of subjects per age group was 43 (range, 32-52). These subjects had been recruited to participate in a study of normal visual acuity.12 About 85% were white. All subjects were born at term(gestational age, ≥37 weeks) with Apgar scores of at least 8, had an uncomplicated neonatal course, were free of medical problems, and, by parental report, were experiencing normal development. Of 545 subjects undergoing refraction, 31 were excluded because of an ophthalmic abnormality (cataract [n = 2], disc anomaly [n = 1], esotropia [n = 2]) or incomplete cycloplegia (n = 26) as judged by the retinoscopist at the time of measurement. The study was approved by the Children's Hospital Committee on Clinical Investigation, Boston, Mass. Written informed consent was obtained from each subject's parent.