Abstract Background: Little is known about the ultrastructure of pili annulati. Objectives: To examine with transmission electron microscopy affected hairs of a family, whose diagnosis had been confirmed in five individuals with scanning electron microscopy, which showed surface undulations with ‘‘curtain-like’’ folding of the hair cuticula and to compare the findings with normal control. Methods: Hairs of two affected patients and one control were embedded in resin and cut lengthwise to produce ultra-thin sections. Results: The normal hair showed a parallel arrangement of dark lines associated with less electron-dense wide bands. Small cavities could be observed, mostly in the dark lines, affected hairs had a large number of cavities, associated or not with the insertion of melanosomes and loss of parallelism of the dark lines. Higher magnification showed a significant loss of this parallelism, resembling ‘‘wood grooves’’. Widened dark lines were observed in some areas. Study limitations: Only a few hairs were examined. Conclusions: The present results suggest that the microcanaliculi of the hair surface, easily found with scanning electron microscopy, may be secondary not only to the cavities seen in the sections but also to the disorder of proteins that form this region, demonstrated by the changes of the cortex dark lines.