A lesion of pseudoxanthoma elasticum at a very early stage was studied by light microscopy and electron microscopy using both tannic acid fixation and the conventional method of fixation. The observations strongly indicated that the primary morphological event arose from alteration in the composition of normal native collagen fiber by way of so-called "elastotically degenerated collagen". Deposition of calcium salts did not appear to initiate degenerative changes in the involved fibers, but merely accompanied the abnormally increased elastic fiber as a secondary phenomenon.