As a part of Colorado's Premature Infant Program, the State Department of Public Health obtained the incidence of prematurity by counties in the state. There was considerable variation among the counties, but one (Lake County) had a much higher incidence than any of the others and, hence, has been the object of more intensive study. Table 1 shows a five-year picture of this county compared with mortality was not proportionately high. The physicians practicing in this area expressed the opinion that babies born there were often small but not premature. The fact that the residents of this county live at an altitude of 10,000-11,000 ft. prompted consideration of the possibility that altitude was responsible for the small babies. In this study detailed medical information was obtained from 180 mothers with the state average for prematurity and neonatal mortality. Although the incidence of prematurity by birth weight was 30.8%, three times