Six macular dystrophy corneas from six individuals were investigated by synchrotron x-ray diffraction. Immunochemical evidence demonstrated that five of the patients had type I macular corneal dystrophy and that one had type II. Analysis of the high-angle x-ray diffraction patterns showed that the intermolecular spacing of macular dystrophy corneal collagen increases with hydration from the dry state in a fashion similar to the intermolecular spacing of normal human corneal collagen. The high-angle x-ray diffraction pattern from all six corneas contained two "extra reflections" not obtained from other human corneas, normal or pathologic. The reflections, which arise from 4.61 and 9.62 A periodic structures, are independent of the type of macular dystrophy. Furthermore, their position is independent of corneal hydration. We propose that a glycosaminoglycan structure is the origin of the unique extra reflections.