The main branch of the left coronary artery of 14 autopsied newborn babies was studied both in light and electron microscopy. Intimal and medial changes of varying severity were found in most cases. The lesions show edema and suggest insudation of the blood solutes into the vessel wall. The edematic phase is followed by proliferation of smooth-muscle and undifferentiated cells, leading to a thickening of the arterial wall. The structure of the lesions resembles the so-called preatherosclerotic changes found frequently in adults. The relationship of the neonatal coronary changes to arteriosclerosis is discussed.