Atherosclerotic lipid deposits found in the core region of fibrous plaques are almost entirely extracellular, but it is not known whether they are derived from necrosis of cells containing accumulated lipid or from direct extracellular lipid accumulation. New evidence pertaining to this question has been obtained through the use of recently developed techniques for preserving and staining lipids in electron microscopy, and through a detailed morphologic and chemical examination of human aortic fibrolipid lesions, which are progenitor lesions for fibrous plaques. The evidence favours a substantial role, perhaps a dominant role, for extracellular lipid accumulation in the formation of the fibrous plaque core region.