Of recent years the cells proliferating in the intima of obliterating endarteritis which is noted in stomach ulcer have been concluded to be connective tissue consisting of fibroblasts. The present author observed electron microscopically seven lesions of obliterating endarteritis at various stages obtained from six cases of chronic stomach ulcer and found that the proliferating cells were all characteristically modified smooth muscle cells. There was no evidence that the cells are either fibroblasts, pre‐existing fibroblasts, maturation of immigrating mononuclear inflammatory cells or endothelial cells. The smooth muscle cells played the leading part in obliterating endarteritis as in intimal thickening due to arteriosclerosis. The arterial smooth muscle cells have many functions such as production of collagen fibers, elastic fibers and basement membrane as well as accumulation of lipid. In conclusion, it may well be said that the arterial smooth muscle cells play the most important role in the pathogenesis of vascular lesions of small arteries including inflammatory lesions morphologicall