The interfibrillar spacing of collagen fibrils was measured at twenty different positions across a macular dystrophy cornea using synchrotron X-ray diffraction. Unlike previous work of this type the cornea had not been frozen for storage. The spacings were all significantly lower than the spacings which existed at similar positions across a normal adult human cornea. This close-packing of collagen fibrils seems to be responsible for the reduced thickness of the central cornea in macular dystrophy. Neither the patient's serum or corneal tissue contained appreciable amounts of sulfated keratan sulfate, this classifies the disease as Type I macular corneal dystrophy.