Scanning electron microscopy studies of human hair defects have been previously confined to descriptions of variation in cuticle morphology and general appearance of the shaft. While the general structure of the hair shaft, i. e., nodes, longitudinal invaginations and twists, occur in regular patterns for given diagnostic categories, there is some question as to the validity of using cuticle morphology as a diagnostic index, since variation within a given subject is usually greater than between subjects. Also, there is a limited amount of information to be obtained from such studies since the metabolically active part of the structure is not visualized. Consequently, the present investigation was initiated to delineate the root sheath through which the emerging hair shaft travels.