History: The Koebner phenomenon is the development of isomorphic pathologic lesions in the traumatized uninvolved skin of patients who have cutaneous diseases.  The phenomenon was described by Heinrich Koebner in 1872.   Psoriasis, lichen planus and vitiligo exhibit true isomorphic phenomenon while pseudoisomophic phenomenon is seen in infectious diseases, e.g. warts, molluscum contagiosum and Behcet's disease and pyoderma gangrenosum.