We performed an immunohistochemical study to try to determine the cellular source of interleukin-8 (IL-8) in psoriatic skin lesions. IL-8 was positively stained in the vast majority of neutrophils but not in the mononuclear cells, macrophages, or keratinocytes. IL-8-positive neutrophils were seen both in Munro's microabcesses in cases of psoriasis vulgaris and in a small spongiform pustule and much larger macropustules of Kogoj in cases of pustular psoriasis. Some IL-8-positive neutrophils were observed in the upper dermis of pustular psoriasis. The staining was considered to be specific because it could be completely blocked by preabsorption with recombinant IL-8. In addition, stimulation of human neutrophils with lipopolysaccharide (LPS) or tumor necrosis factor-alpha (TNF-alpha) for 18 h induced IL-8 production in vitro. In our study, IL-8 was expressed in the neutrophils of psoriasis, suggesting that neutrophils are one of the sources of IL-8 in psoriasis. The expression of IL-8 and the influx of neutrophils led us to speculate that the IL-8 autocrine and/or paracrine system functions in the formation of the microabcesses and pustules in proriasis.