The medical writer, Thomas McKeown, can justifiably claim to have been one of the most influential figures in the development of the social history of medicine during the third quarter of the twentieth century. Between 1955 and his death in 1988, he published a stream of articles and books in which he outlined his ideas about the reasons for the decline of mortality and the ‘modern rise of population’ in Britain and other countries from the early eighteenth century onwards. Although McKeown's main aim was to deflate the claims made by the proponents of therapeutic medicine, his publications have sparked a long and protracted debate about the respective roles of improvements in sanitation and nutrition in the process of mortality decline, with particular emphasis in recent years on the impact of sanitary reform in the second half of the nineteenth century. This article attempts to place the debate over the ‘McKeown thesis’ in a more long-term context, by looking at the determinants of mortality change in England and Wales throughout the whole of the period between c. 1750 and 1914, and pays particular attention to the role of nutrition. It offers a qualified defence of the McKeown hypothesis, and argues that nutrition needs to be regarded as one of a battery of factors, often interacting, which played a key role in Britain's mortality transition.