In biopsies of 16 patients (mean: 5.2 years) with acute systemic onset juvenile idiopathic arthritis (SOJIA), we analysed the initial cellular events during the characteristic cutaneous rash for composition of the infiltrate and for expression of activation markers on epithelial and endothelial cells. Despite the fleeting nature of the rash, there was a characteristic infiltration of neutrophils and monocytes, accompanied by a marked expression of endothelial adhesion receptors. In addition, we found a general activation of the cutaneous epithelium reflected by the expression of the pro-inflammatory S100-proteins - myeloid-related protein 8 (MRP8) and MRP14. In responders to therapy, follow-up biopsies showed a complete normalization of these inflammatory parameters, whereas non-responders presented with continuous signs of activation. In conjunction with the high level of epithelial activation, we detected an infiltrate of leucocytes within epithelium of sweat gland ducts during active SOJIA. Such a pattern has not been described for other inflammatory skin diseases nor did we find it in biopsies from nine patients with acute urticaria. It was accompanied by exclusive expression of MRP8, but not MRP14 by the secretory cells of sweat glands. Because MRP8 and MRP14, released by epithelial cells, exhibit pro-inflammatory effects on endothelial cells and leucocytes, the particular expression pattern of MRP8 and MRP14 in SOJIA is likely to represent a decisive early constitutive component in this inflammatory disease. Their differential expression further points to distinct roles of the individual molecules in inflammatory processes.