Several hypotheses have been offered to explain the occurrence of arteriosclerotic calcification but the mechanisms involved are still not well understood. Using a combination of electron microscopy and immunohistochemistry, atherosclerotic plaques from human arteries as well as atherosclerotic-like lesions from aortas of apo-E-deficient mice were examined to identify cell type(s) associated with calcification. Electron microscopic analysis showed that, in human atherosclerotic plaques, chondrocyte-like cells were present in areas surrounding the necrotic cores. In these areas, some smooth muscle cells displayed features of their transdifferentiation into chondrocyte-like cells. Immunohistochemical analysis confirmed that smooth muscle cells with a reduced content of α-smooth muscle actin expressed Sox-9. Destruction of chondrocytes resulted in the accumulation of numerous membrane-bound vesicles in the extracellular space. Membrane-bound vesicles originating from chondrocytes were found to undergo calcification. Similar processes were found to occur in atherosclerotic-like lesions in apo-E-deficient mice. These observations suggest that transdifferentiation of smooth muscle cells into chondrocytes contributes to atherosclerotic calcification. Copyright © 2005 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.