The lesions of porcine proliferative enteritis were studied by light and electron microscopic techniques in feeder pigs, fattening hogs, bred gilts, sows, and boars. The characteristic microscopic feature common to all age groups was proliferation of immature crypt epithelial cells, primarily in the ileum and the distal part of the jejunum. Similar changes were also observed in the midjejunum, cecum, and colon of a new swine. The earliest detectable microscopic lesion was focal proliferation of crypt epithelial cells with accompanying inflammation of the lamina propria and leukocytic exudate within affected crypt lumina. Lesions progressed to diffuse crypt cell proliferation, elongation of crypts, and loss of villi. Immature epithelial cells contained variable numbers of intracytoplasmic, nonmembrane-bound, curved organisms resembling Campylobacter sp bacteria. Similar organisms were within phagolysosomes of macrophages in the lamina propria and within the cytoplasm of crypt epithelial cells undergoing mitosis. Campylobacter sputorum subsp mucosalis was isolated from the ileal mucosa in 30 of 58 swine.