In this study, we compare cross-sectional growth in two Nilotic pastoralist populations, the Turkana and Karimojong of northern East Africa. Until the middle of the 18th century, these two groups constituted one population, and their genetic affinities consequently are assumed to be strong. Known differences in their subsistence practices as well as their recent cultural history allow us to consider the genetic vs. environmental determinants of their growth patterns. Growth in stature of the two groups is remarkably similar, whereas the pattern of ponderal growth varies, particularly in infancy and adolescence. Both groups demonstrate the linear adult physique that is characteristic of savanna-dwelling peoples in East Africa, but the Karimojong are heavier and fatter after early childhood. Linearity of physique is related to narrow skeletal breadth, a trait that emerges early in childhood among the Karimojong and is strongly correlated with weight: this finding supports the climatic hypothesis of human body shape and size. Extreme leanness, determined from body mass index, is correlated with fat stores, which are only weakly correlated with weight, height, or skeletal breadth. Linearity of physique appears to have a substantial genetic component, whereas fatness is hypothesized to relate to differences in subsistence strategies and exposure to environmental and cultural stress.