One common feature of atherosclerotic plaques is the denudation of the endothelium covering the plaque and subsequent failure of endothelial regrowth in contrast to a marked proliferation of neocapillaries arising from the vasa vasorum within the medial wall. Previous studies in vitro have demonstrated the ability of leukemia inhibitory factor (LIF) to potently inhibit aortic endothelial cell growth while only slightly inhibiting the growth of adrenal cortex capillary endothelial cells. This selective effect of LIF on endothelial cells from different sources suggests that it may play a role in the failure of endothelial regrowth in atherosclerosis. Sections of human carotid endarterectomy samples were examined by in situ hybridization for LIF mRNA expression. In one-third of the samples (4/12) examined, cells within the atherosclerotic plaque exhibited LIF expression. Immunohistochemistry of serial sections suggested that the LIF-positive cells were activated macrophages. These results suggest that LIF may play a role in the pathogenesis of atherosclerosis, particularly the denudation of the large vessel endothelium.