Characteristic moniliform hairs of monilethrix were ultrastructurally examined. By scanning electron microscope, nodes and internodes were seen alternating on the affected hair; the nodes were normal in appearance and thickness, while the internodes were thin and showed ridges and flutes. By transmission electron microscope, the cross sections of the internodes revealed wrinkling of the hair cuticular cells and a reduced number of the cortical cells. Cross sections of the cortical cells per se showed a similar size and a normal keratin pattern in both nodes and internodes, compared with those of control hairs from normal individuals. From these findings, the internodes seemed to be the pathological portions of the moniliform hair, and such abnormal thinning of hair shaft might be caused by a periodical dysfunction of the hair matrix, especially in the hair cortex.