In examining the prismless enamel (PL) distribution from the inner to surface layers of a human incisor with hypoplastic perikymatas with enamel pits, we determined the structural patterns and mineralization values by microradiographic, optical, and scanning electron microscopic observations. The PL containing abnormal prisms ranged from 20 to 250 micrometers in thickness. The alternated layers of the PL and prismatic enamel, in parallel with the Retzius lines, showed a roughly zigzag pattern based on the main orientation of enamel crystals. Such a pattern suggests an alternation of the disappearance and reappearance of Tomes' processes, which may have strong restorative and reactive capacities. In the mineralization values determined by microradiography, the PL was higher in the well-marked lines of Retzius, while the PL under the hypoplastic enamel pit and an area within the thick PL showed hypomineralization. The backscattered electron images indicated that the fine laminate striations and abnormal prismatic rows within the PL were hypomineralized, and the PL in the inner enamel showed a lower mineralization than the adjacent prismatic layer. Thus, the disappearance of Tomes' processes, which form prismless structures, may be unrelated to changing the mineral amount in amelogenesis.