An examination of recent Colorado vital statistics records was performed. The records included 194,526 live births and 3,100 infant deaths occurring during the years 1969-73, and some 466,000 live births and 71,000 infant deaths occurring during 1960-71. Birth weights at high altitude (2,744 to 3,100 m) were found to be reduced owing to retardation of intrauterine growth rather than preterm delivery. Despite decreased infant mortality at all altitudes in Colorado during the last 20 years, the infant mortality rate at high altitude remains almost twice that seen in Denver. The elevated mortaltiy at high altitude occurred within 28 days after birth, primarily in preterm (less than 38 weeks) infants. It appears that high altitude, possibly throuth the mechansif of exaggerated fetal hypoxia, retards fetal growth and increases the mortality rate of preterm infants.