An examination of the pathological features of 12 consecutive cases of sudden death in infancy was undertaken with particular reference to the lung changes used a combined radiological and pathological study of formalin vapour-inflated lungs. Certain common features in the cases in which there are no post mortem changes to explain death (typical cot deaths) are described and compared with cases with an acceptable cause of death and with control cases. It was concluded that severe dehydration is a potentially avoidable factor underlying cot deaths and many other sudden infant deaths with no explainable causes; that petechial haemorrhages are a constant feature of cot deaths and indicate a respiratory/hypoxic mode of death; that absence of petechiae indicates some other explanation for death. It is suggested that the minor respiratory or other infections which are usually present at autopsy in cot deaths may sometimes exaggerate a preexisting state of dehydration and that the dehydration may contribute more directly to death than the infection.