A decrease in birth weight occurs at high altitude, but its relationship to infant mortality is unclear. We examined Colorado vital statistics recorded from 1979 through 1982 to determine whether high altitude increased infant mortality and whether decreased birth weight contributed to the mortality observed. Retardation of intrauterine growth reduced birth weight and doubled the frequency of low-birth-weight infants from the lowest (915 to 1523 m [3000 to 4999 ft]) to the highest (greater than or equal to 2744 m [greater than or equal to 9000 ft]) altitude in the state. Low birth weight increased mortality risk, but the mortality risk of low birth weight was decreased at high compared with low altitudes, resulting in similar infant mortality rates throughout the state. This finding differed from that of 1969 through 1973 when infant mortality doubled at high altitude. A 46% infant mortality reduction had occurred statewide over the ten years due chiefly to decreased mortality risk for preterm low-birth-weight infants. This reduction, particularly pronounced at high altitude, might have been due to better identification and transport of high-risk pregnancies to hospitals with tertiary neonatal treatment centers.