Laboratory apparatus was designed to administer controlled stress by rubbing 53 samples of normal appearing scalp hair, some taken from healthy subjects and some from patients with trichorrhexis nodosa, in other hair. The characteristic picture of trichorrhexis nodosa was produced in all specimens. The two patient groups did not differ significantly in the number of rubs required to produce the condition. Therefore, trichorrhexis nodosa may be more often related to the amount of trauma received by hair shafts rather than to some inherent chemical or structural defect.