The Journal of Dermatology VoJ. 24: 583-586, 1997 Koebner Phenomenon in an ANCA-Positive Patient with Pyoderma Gangrenosum Massimo Papi, Biagio Didona, Luca Massimo Chinni, Tommaso Cobello, Cinzia Mazzanti, Ornella De Pita and Rino Cavalieri Abstract A male with pyoderma gangrenosum is reported. The clinical and histological features were typical. The initial lesions resolved with characteristic cribriform scars. A few days after the complete recovery, he developed several necrotizing focal lesions localized to the scarred areas. A further histological examination revealed a granulation tissue rich in neu- trophils and signs of necrotizing vasculitis. We found a high titer of circulating perinuclear antineutrophil antibodies (p-ANCA), which are a serological marker for various systemic diseases. An immunological circulating factor has been repeatedly suggested to be the "primum movens" of pyoderma gangrenosum. We discuss the unusual clinical presenta- tion interpreted as a Koebner phenomenon and the possible role of immune factors in en- hancing circulating-endothelial cell interactions in relation to the pathogenesis of pyoder- ma gangrenosum. Key words: pyoderma gangrenosum; Koebner phenomenon; ANCA; vascular neoformation Introduction The Koebner phenomenon (KP) is the development of pathologic isomorphic le- sions in previously uninvolved skin of a pa- tient with cutaneous disease, following a me- chanical trauma or other noxious stimulus (1, 2). It is a distinguishing feature of psori- asis, but it has been described in other cuta- neous disorders (3). Similar phenomena are seen in the pathergy of Behcet disease. Recently Chan has interpreted the devel- opment on scar sites of two immune-com- plex mediated vasculitis as Koebnerization (4). The easier local deposition of im- munoreactants can be explained by the trauma-altered cutaneous vascularity. This Received April 5, 1996; accepted for publication May 23,1997. Istituto Dermopatico Immacolata-I.R.C.C.S., Via Monti di Creta, 104,00167 Rome, Italy Reprint requests to: Massimo Papi, M.D., Via Monti di Creta, 104,00167, Rome, Italy. phenomenon could be related to the scar condition known as "locus minoris resisten- tiae" (5). We report the case of an antineutrophil cytoplasmic antibody (ANCA)-P9sitive pa- tient with pyoderma gangrenosum (PC) who developed eruptive spotty necrotizing lesions on atrophic areas healed from previ- ous lesions of PC. Case Report A 51-year-old male patient presented with a two-month history of painful ulcerative lesions of the right leg. He had been diagnosed with ulcerative colitis two years earlier and was taking 1 g salazopyrine daily. On examination a large, circumscribed, ulcer- ative lesion of the external surface of the leg with an active, violaceous, edematous upper bor- der was present (Fig. 1). The lower part of the lesion had healed with a cribriform aspect. A similar evolving lesion of the right ankle had re- cently healed with residual atrophic scarring. Histological examination of the advancing