The intestinal tissues from 11 pigs orally inoculated with Escherichia coli (E. coli, 0139:K12:H1) were examined by transmission electron microscopy. The colonization of E. coli along the small intestinal mucosa was found in seven principals without any major changes in the enterocytes from day 2 to day 7 after inoculation when the experiment was terminated. Lesions of vessels of the intestinal mucosa could be detected as early as two days after inoculation and persisted until the experiment was terminated. Lesions consisted of endothelial swelling and vacuolation, subendothelial fibrin deposition, perivascular edema, microthrombus formation, endothelial proliferation, and necrosis of the tunica media. The possible pathogenesis of the disease is discussed.