A growing body of evidence has indicated that T-cell-mediated immunity plays an important role in triggering and maintenance of psoriatic lesions. In this review we present our own experimental results as well as those from the literature related to the pathomechanism of the development of inflammatory changes in psoriatic lesions. First of all it is important to acknowledge the fact that psoriatic lesions are not uniform as assumed by many authors but that they are actually rather heterogeneous both clinically and histologically even within the same plaques. Lymphokines produced by activated T cells in psoriatic lesions have a strong influence on the proliferation of the epidermis, whose stimulated kertinocytes released several cytokines, which in turn enhance the activation state of T cells. Thus, they form a vicious cycle, a T-cell-mediated inflammation-sustaining loop. Although the interaction between T-cell-mediated immunity and epidermal keratinocytes may well explain the maintenance of background "chronic" inflammatory changes diffusely observed throughout psoriatic lesions, it is not enough to explain the island-like, "acute" inflammatory changes observed within and at the border of the plaque lesions. Characteristic neutrophil accumulation under the stratum corneum can be observed in the highly inflamed and therapeutically recalcitrant areas of psoriatic lesions. They are chemotactically attracted and activated there by synergistic action of chemokines, IL-8 and Gro-a released by the stimulated keratinocytes, and particularly C5a/C5a des arg produced via the alternative complement pathway activation possibly on the surface of corneocytes. In this review, we emphasize that the accumulation of neurophils is not simply a passive event. We think that those stimulated neutrophils are able to influence not only the growth and differentiation of epidermal keratinocytes but also the activation-state of T cells by aberrant expression of HLA-DR on their surfaces as well as by their effects. These T cells in turn influence the transepidermal neutrophil migration through the effect of their lymphokines on the keratinocyte production of pro-inflammatory mediators including C3. Therefore, we propose a neutrophil-associated inflammation-boosting loop that may well explain the localized "acute" inflammatory changes scattered over the "chronic" psoriatic plaques as well as in the acutely inflamed lesions of pustular psoriasis.