The plucked hairs and biopsied hair follicles of the scalp were obtained from a female patient with monilethrix. By scanning electron microscopy, the plucked hairs showed a typical moniliform feature composed of alternated nodes and internodes. By computer stereography, reconstructed three-dimensional models of in vivo hair structures showed that the diameter of hair shaft was partially reduced in the keratogenous zone and that the reduction was severe in the hair cortex and cuticle layers but mild in the inner root sheath. By transmission electron microscopy, a significant degeneration of hair matrix cells was found, and a zig-zag disarray of cortical tonofibrils and invaginations of the hair cuticle cells into the cortex were noticed in the suprabulbar portion. In some hairs, the hair bulbs showed no degeneration, but a degeneration of cortical cells and invaginations of hair cuticle were seen in some portions in the developing zone of the cortex. This suggests that not every hair matrix is damaged in a synchronized fashion, but individual hairs are affected independently. Furthermore, in all the hair and hair follicles examined, cytoplasmic vacuolations were always seen in the various layers, and abnormal formation of tonofibrils was often observed in the cortex. In monilethrix, a cell abnormality may inherently be present in the hair tissue and, when such abnormality occurs severely, the cortical cells are particularly affected in the hair matrix. This seems to result in a decrease in number of cortical cells and thinning of the hair shaft.