The amorphous areas, which are gray in sections stained with hematoxylin and eosin and which are a prominent feature in the coronal dentin of the more severely affected cases of regional odontodysplasia, were studied ultrastructurally and with an electron probe and a Vickers microhardnesss tester. The ultrastructural findings confirmed previous histochemical evidence that the amorphous material consists of glycosaminoglycans and represents collagen-free dentinal matrix, that is, ground substance. Phosphoproteins may also present. The ultrastructural examination of nondecalcified material and electron-probe and microhardness tests supported previous microradiographic evidence that the amorphous areas are more heavily mineralized than normal dentin. The boundaries of the amorphous areas were found on ultrastructural examination to be distinct but very irregular.