This study describes in detail the enamel defects in a patient in whom the diagnosis of Morquio syndrome (muchopolysaccharidosis IV) has been supported both biochemically and roentgenographically. These defects are an apparently constant feature of the Morquio syndrome, and are therefore important aids to its differential diagnosis. The defects consist of abnormally thin enamel that is rough because of the numerous minute pits on its surface. The thinness of the enamel results in altered shape and discoloration of the teeth and in their being spaced. Apart from these defects, the enamel is histologically normal and is of normal hardness and radiodensity.