The three-dimensional structure of the collagen network of normal mucosa, hyperplastic polyps, tubular adenoma, villous adenoma and adenocarcinoma of the human large intestine was examined by scanning electron microscopy after cell-maceration with a low temperature NaOH solution. In the normal large intestine, the surface of the sub-basal laminar collagen sheath lining the crypts was generally smooth. In hyperplastic polyps, the crypts extended and the sub-basal collagen network protruded from the surrounding normal tissue. The sub-basal laminar collagen sheath was more densely arranged near the openings of the crypts and on the luminal surface than in deeper regions of the crypts. In tubular adenoma, the tumor glands showed meandering, bending, and divisions. The sub-basal laminar collagen sheath was composed of densely packed collagen fibrils. In villous adenoma, the sub-basal laminar collagen sheath formed foliate or linguiform villous cores projecting sharply into the lumen. In adenocarcinoma, the sub-basal laminar collagen sheath exhibited a dense arrangement of fibrils in the central region of the tumor. Toward the marginal region, the collagen sheaths surrounding tumor glands became thinner, and at the edge of the marginal region, basket-like collagen sheaths were observed around individual tumor cells infiltrating the interstitium. Such a tumor lesion was frequently surrounded by a thick collagen fiber zone.