Two patients with dermatitis traumatica due to repeated biting were studied. Biopsies from the grossly thickened traumatized areas were compared with adjacent normal skin. The altered epidermis was 5–8 times thicker than normal and the corium was more than twice as thick as normal. Blood vessels and nerves appeared to be increased in number and larger in diameter in the traumatized areas. Hair follicles in the traumatized area were enlarged and contained coarse terminal hairs in contrast to lanugo hairs in the normal skin. A relative absence of inflammation in the traumatized skin contrasted with the inflammatory changes seen in lichen simplex chronicus and prurigo nodularis. The inflammatory response, apparently resulting from trauma in these latter diseases, may indicate an abnormal host respons