In this article implications of season of birth for child health in Spain during the demographic transition are explored. Making use of the Aranjuez data base, the authors look at the seasonality of births and its heterogeneities, finding a nearly universal prevalence of late spring and early summer conceptions that is most evident with last-born children. Levels and causes of childhood mortality are strongly related to the season of birth, with those born between April and October showing lower likelihood of survival. These findings can be explained by the way infant feeding, age and season intersect. These results have implications for the relative efficiency of different demographic regimes.