The special characteristics of mortality in Spain in the nineteenth and early twentieth centuries, particularly with respect to the high levels of childhood mortality, led us to attempt to decipher the internal structures of this phenomenon. This study focuses on the role of early childhood mortality (ECMR, ₄q₁) and infant mortality (q₀), with particular reference to the former, as essential variables in explaining the decline of mortality in Spain. Thus, the structure of mortality in the early years is emphasised as a key indicator of the evolution and modernisation of the population, given the differences that have been observed in different countries and contexts, and the connection which exists between infant and child mortality in the process of demographic transition. After a comparison of the levels of mortality in Spain with those of other countries, which highlights the importance of studying child mortality, an analysis is carried out using the data available for Spain and its provinces, revealing important differences in the internal structures of childhood mortality. These differences are reflected both in the geography of the phenomenon and in the differences between urban and rural environments, hence the importance of this type of analysis in explaining the diversity of experience and the regionalisation of the demographic modernisation in Spain. Moreover, it is shown that the decline in childhood mortality varied between regions and between rural and urban areas; consideration is also given to the variations in childhood mortality according to the size of the urban populations. Copyright © 2000 John Wiley & Sons, Ltd.