We examine the birth order effects on health status for a sample of children aged 1–18 years in South Africa. Using a mother fixed-effects specification, we observe children’s height-for-age z-score decreases with birth order. We investigate potential mechanisms underlying the birth order effect including those related to biology, parental preferences, and resource dilution. We also look at whether these effects are due to selection into families of different sizes. We find that the magnitude of the effect is larger in poorerand rural households and in larger families – suggesting that the birth order effect is largely due to resource dilution in economically constrained households.