Low-angle synchrotron X-ray diffraction has revealed clear and consistent changes in the molecular structure of alpha-keratin of hair in insulin-dependent diabetes (IDDM) both for human IDDM subjects and for baboons with streptozocin induced diabetes. These changes in both meridional and equatorial intensity distributions are fully explained by a newly developed hexagonally packed model for keratin which locates the modification produced in hair in IDDM in the labile structure of the matrix at established intermediate filament linkage sites. The nature of the extracellular bonding suggests that the change is endogenous, occurring via the blood during the aggregation of the IFs in the follicle. The reproducibility of these changes indicate that hair may represent an easily accessible tissue for the study of how hyperglycaemia can modify extracellular matrix materials which lead to diabetic complications.