A case of annular pustular psoriasis is described. The annular zone of the lesion provides a model for studying the dynamics of intraepidermal accumulation of polymorphonuclear leukocytes in psoriasis. It could be shown that the unstimulated accumulation of polymorphonuclear leukocytes was limited to the clinically involved skin. The appearance of polymorphonuclear leukocytes proved to be associated with acanthosis. The micropustule formation following topical application of leukotriene B4 was markedly decreased in lesional, prelesional and postlesional skin, compared to the response in the clinically uninvolved skin of patients with chronic plaque psoriasis. Although leukotriene B4 might have an initiating role in the intraepidermal accumulation of polymorphonuclear leukocytes other factors are a conditio sine qua non for the peripheral spreading of the lesion in annular pustular psoriasis.