We investigated the morphological characteristics, in histological and immunohistochemical studies, of necropsied coronary arteries from 7 patients who died between 2 days and 11 months after Palmaz-Schatz stent implantation. At 2 days, the stent struts locally depressed the arterial wall, causing thinning and stretching of the atherosclerotic plaque-free wall segment. The fibrous cap seems to have ruptured, but compression of atheroma by the stent wires was not documented. There were fibrin deposits and infiltration of blood elements, mainly leukocytes and macrophages, around the stent struts. Light microscopy clearly revealed a new intimal thickening at 1 month, consisting of several layers of smooth muscle cells within the stented vessel. Thereafter, significant proliferation of pleomorphic smooth muscle cells and an increase in extracellular matrix were observed on the intimal side of the stent struts, predominantly around the circumference of the lumen at the stented site. Arteries from patients with restenosis who died more than 1 month after stenting demonstrated an apparent T lymphocyte and macrophage infiltration around the stent wires. Some of them featured the appearance of multinucleated giant cells, in close contact with the stent wires. There was occasional granulated tissue formation, which often extended into the media and adventitia, seen adjacent to the stent struts. Arteries without restenosis showed the smooth muscle cells to be spindle-shaped and ordered along the luminal surface. In these cases, the intercellular space contained abundant collagen fibers, and no significant inflammatory cells.These findings suggest, that in some cases, stent implantation evokes remarkable inflammatory reaction of arteries to the metallic stent as a foreign body, which may lead to significantly greater intimal proliferation than do the balloon injured segments might.