The reason for the reported decline in the proportion of elderly (>60 years) persons at high (>2,456 m) compared with low altitude (<1,376 m) in Colorado was unknown. We hypothesized that adverse effects of high altitude on the elderly, particularly those with heart and lung diseases, prompted their migration to lower elevations. Colorado census data indicated that selective out-migration occurred from high to low altitude among the elderly. Interviews (n=833) in high- and low-altitude Colorado towns revealed that the elderly were unique in that they moved down for reasons of poor health and that for the majority (81%) ill health meant heart and lung diseases. Elderly migrants from high altitude reported heart and lung diseases more frequently than those remaining and cited improvement in symptoms at low altitude. We suggest that symptoms of heart and lung disease are exacerbated with advancing age at high altitude and influence choice of residence.