In this study were observed the effects of two widely differing environments on the growth and maturation of children from a presumed genetically homogeneous Ethiopian population. Major environmental differences included altitude above sea level, temperature, probably rainfall and humidity, together with the incidence of infectious disease. The results indicate that highland children, particularly boys, are taller, heavier and bigger in most physical dimensions than are lowland children. In both groups skeletal maturation is retarded (by American White standards) during later childhood; this retardation is more marked in lowlanders. In both groups, however, there is marked acceleration of skeletal maturation during early puberty. Haemoglobin values increase much more rapidly in highland children, but surprisingly, differences in chest dimensions are not particularly marked. It is concluded that hypoxia of the degree found in the high-altitude group (approximately 3000 m) is not sufficient to affect adversely the growth of children. On the other hand, the increased incidence of infectious disease in the 'lowlands' (approximately 1500 m) and possibly the raised ambient temperature, may restrict growth and maturation of children living in this environment. Thus, in contrast to the situation in other high-altitude parts of the world, the highlands in Ethiopia appear to be more favourable to growth than the lowlands.