The diagnosis of structural abnormalities of the hair shaft is of clinical relevance, since such abnormalities may indicate underlying metabolic disorders, or may be associated with other diseases. Many of the abnormalities present themselves as alopecia and are due to greater fragility of the hair shaft. An outline of some of the most common hair shaft abnormalities is given, as shown by patients with trichorrhexis nodosa, pili torti, "uncombable hair" and pili torti et canaliculi.