A characteristic feature of the early and active psoriatic lesion is the intraepidermal penetration of polymorphonuclear leukocytes (PMN) with the formation of intraepidermal micropustules of Kogoj and microabscesses of Munro, localized in the stratum corneum. In pustular psoriasis the accumulation of PMN dominates the clinical picture. During the last decade our insight into the regulation of the migration of PMN has increased. The properties of the circulating blood PMN and the biochemical nature and biological effects of mediators of inflammation have been studied in detail. Several groups have demonstrated that the transdermal and epidermal migration of PMN following challenge with a chemoattractant is slightly diminished in the clinically uninvolved skin and profoundly decreased in the lesional skin of psoriatics, compared to the responsiveness in normal controls. The molecular nature of this process of tachyphylaxis has not yet been established. However, it has been hypothesised that some mediators of inflammation, cytochrome P-450, endothelial cell function, and protease inhibitors might be of relevance in this respect. This report is a review of the literature on transdermal and epidermal migration of PMN in normal and psoriatic skin. The aim of this review is to clarify further the position of the PMN in the pathogenesis of psoriasis and its role as a target for antipsoriatic treatments.