Background: The Koebner phenomenon is defined as 'the development of psoriasis at sites of traumatized skin'. The 'all-or-none principle' means that, if psoriasis occurs in one area of injury, all injured areas develop psoriasis or vice versa. The aim is to demonstrate the concordance of patients with the 'all-or-none principle when a standard method of trauma is employed. Methods: Sixty-two patients with psoriasis were enrolled in the study. Demographic data and disease characteristics were recorded. The medial aspects of both forearms, devoid of lesions, were pricked using two sets of five 30-gauge needles at an angle of 30, with 2-cm intervals. On days 14 and 28, the patients' forearms were checked for the presence of a typical psoriatic plaque of white scales on an erythematous papule. Results: On day 28, 45 patients (72.5%) had a negative Koebner response in all prick sites whereas 1 patient (1.6%) had psoriatic papules in 10 out of 10 prick sites. The rest of the patients (n = 16, 25.8%) had between 1 and 9 papules in number. Conclusion: Using standard methods of trauma, it is possible to induce psoriasis lesions as a Koebner response but this response is not always in concordance with the 'all-or-none principle previously described. Copyright (c) 2007 S. Karger AG, Basel.