It has long been believed that neutrophils have main role in the pathogenesis of Behcet's disease. In contrast, a needle reaction which is specific phenomenon in this disorder is the delayed type reaction and reveals the predominant infiltration of lymphocytes. In order to clarify these controversial problems, histopathological examination of the biopsied specimens of erythema nodosum (EN) and oral aphthous ulceration (OA), was carried out. Predominant mononuclear cell (lymphocyte and macrophage) infiltrations were noted at the early stage of EN (within 24 hours after an emergence of EN) and the preulcerative stage of OA. No neutrophils were observed at this stage. Many pyroninophilic cells were also noted at the same lesion, suggesting lymphocyte activation at the site of lesions. Then, neutrophil appeared as disease progressing. The mononuclear cell infiltration in the early stage of EN and OA, and the presence of pyroninophilic cells may suggest that these cells have a essential role for the onset of lesions in Bechet's disease. Neutrophil chemotaxis may then occur in the site of EN and OA by unknown immunological mechanism.