In the present study we have investigated the early histopathologic as changes occurring in the Koebner reaction induced by traumatic injury in uninvolved skin of 23 psoriatic and 7 non-psoriatic control patients. A punch biopsy of the injured area was performed after 2-3 (15 cases) or 7 days (8 cases). As a trauma, instead of the classic sellotape stripping, needle scarification was used. A peculiar histological feature of the skin biopsies of 13/23 psoriatic patients (56%) was a keratinocyte hyperplasia leading to a "papillary" projection into the upper dermis, just beneath the scarification. The papillary projection was associated with the expression of intercellular adhesion molecule-1 (ICAM-1) in the keratinocytes of 9/13 cases (70%) and with the presence of peri-papillary aggregates of CD68(+) cells in 10/13 cases. In the upper dermis, tenascin was markedly expressed in 12/13 cases. Moreover, in one third of the cases, just beneath the scarification, there was reabsorption of the epidermal basal membrane as documented by a marked reduction of collagen type IV and laminin content. These histopathological alterations were detected in 6/15 psoriatic patients whose skin biopsy was taken 2-3 days after scarification, in 7/8 after 7 days, and in only 1/7 non psoriatic controls. Our results indicate that needle scarification can be a suitable method to study the early events occurring in trauma injured psoriatic skin.