In New Mexico, where inhabited areas vary from 914 to over 2135 m above sea level, we compared age-adjusted mortality rates for arteriosclerotic heart disease for white men and women for the years 1957-1970 in five sets of counties, grouped by altitude in 305-m (1000-foot) increments. The results show a serial decline in mortality from the lowest to the highest altitude for males but not for females. Mortality rates for males residing in the county groups higher than 1220 m in order of ascending altitude were 98, 90, 86 and 72 per cent of that for the county group below 1220-m altitude (P less than 0.0001). The results do not appear to be explained by artifacts in ascertainment, variations in ethnicity or urbanization. A possible explanation of the trend is that adjustment to residence at high altitude is incomplete and daily activities therefore represent greater exercise than when undertaken at lower altitudes.