AbstractAnalysis of Peruvian census materials shows an increase of neonatal mortality with altitude. This increase is analagous to, but greater than, that known to obtain in the United States population. Neonatal mortality in the highlands of Peru averages from 50 to 60 deaths per 1,000 live births, about double that of the lowlands. The association of neonatal mortality with elevation appears partially diminished when other areal differences are considered.