Examination of the small intestine of pigs with proliferative haemorrhagic enteropathy showed changes consistent with defects in vascular permeability. Early in the disease there were many eosinophils and distension of lacteals and intercellular spaces with proteinaceous material. Later the predominant features were red blood cells and exudate in tissue spaces. This was most severe and extensive at the tips of villi which were covered by a cast of cells and fibrinous exudate. Adenomatous intestinal mucosal cells contained organisms that were free within the apical cytoplasm and were morphologically identical with those seen in the related disease, porcine intestinal adenomatosis. Also these bacteria were seen free in the subepithelial mucosal area, in blood vessels and within membrane-bound vesicles in phagocytic cells in the mucosa and its blood vessels. Mast cells were prominent in some areas as were thrombosed vessels.