The ultrastructure of calcium crystals and their aggregates was studied in thin sections of human arteries, obtained at autopsy. Both lightly and heavily calcified regions were examined. The crystals were similar in tissues fixed with ethanol, formaldehyde, or formaldehyde and osmium tetroxide. The sizes of the pleomorphic crystals varied considerably. Some were granular, others needlelike. The largest crystals were plate shaped and measured up to 200 m μ in length, 80 m μ in width, and 8 m μ in thickness. The morphologic features of the crystal aggregates and several other types of deposits are described, and their significance is discussed.