Calcified deposits in the tunica media of the human aorta have been studied in 128 cases by light microscopy and by electron microscopy and analytical methods in selected samples. Although dissolved and not visible in routine histology with alum hematoxylin stains, such calcification can be clearly seen after methylene blue staining in the form of unstained refractile particles of 1-2 microns size. These are found between the elastic laminae chiefly in the inner two-thirds of the media and appear at about age 20. By X-ray diffraction supported by energy dispersive X-ray analysis, they have been identified as whitlockite (Ca,Mg)3(PO4)2. Statistical analysis shows a significant increase in numbers with age and significant differences in severity related to county of origin but no differences between sexes or races and no correlation with deaths related to cardiovascular diseases. Among various substructures of the aortic wall, no unique crystal precursor was identified. Possible etiologic factors and clinicopathologic significance are considered.