Spatial disparities in the prevalence of heart disease are frequently explained in terms of adult lifestyle factors (e.g. diet, smoking, alcohol consumption, stress, exercise, etc.). However, in recent years a number of researchers have suggested an alternative mode of explanation: namely, that the risk of heart disease in adult life may be influenced either by factors related to living conditions shortly after birth or by foetal development before birth. These propositions may be referred to collectively (if loosely) as the "perinatal" hypothesis. Much of the early research on the perinatal hypothesis was prompted by observed ecological correlations between adult heart disease mortality and infant mortality within the same age cohort several decades previously. However, correlations of this type have so far been reported for a limited number of countries, raising the question as to what extent they are replicable in other geographical contexts. This paper reports the results of an ecological analysis of adult heart disease and infant mortality in the Republic of Ireland. The first part of the paper traces the origins and evolution of the perinatal hypothesis and identifies some of the major issues which have arisen in the literature. The middle sections of the paper report the results of an empirical study using data on infant mortality and deaths from ischaemic and other forms of heart disease in the Republic of Ireland. The final part of the paper discusses the extent to which these findings provide support, or otherwise, for the perinatal hypothesis.