Langerhans cell histiocytosis (histiocytosis X) is a clonal proliferative disorder of the dendritic antigen-presenting Langerhans cells (LCs) in the skin and other organs. The LCs show phenotypical features of aberrant activation and/or differentiation, but there is no pronounced proliferative activity of the cells, in spite of their strong expression of proliferation markers. To get insight into the molecular pathogenesis of these cellular alterations, we investigated the expression of the master switch gene p21, which encodes a cyclin-dependent kinase inhibitor that regulates the growth and differentiation of cells. Skin biopsies from 5 patients (3 infants, 2 adults) with Langerhans cell histiocytoses were incubated with moAbs against the p21 protein, the tumor suppressor gene p53, and the proliferation marker PCNA using light and electron microscopical immunolabelling. In all biopsies, the CD1a-positive tumour cells showed nuclear stainig for p21. This reaction was especially found in those tumour cells which demonstrated the morphologic features of cellular activation. The expression of p21 was paralleled by positive immunostaining for PCNA. In contrast, no expression of p53 could be observed in the tumour cells. In biopsies of normal skin (n=5), the CD1a-positive epidermal LCs expressed neither p21, nor p53 and PCNA. Taken together, the abnormal LCs of Langerhans cell histiocytosis, but not normal LCs express the p21 gene by a p53-independent pathway. This molecular pathway has very recently been shown to inhibit the PCNA-dependent proliferation and growth of cells by inducing their differentiation. Our data thus imply that the cellular behaviour of the LCs in Langerhans cell histiocytosis is due to a p21-mediated aberrant differentiation and activation of the PCNA-positive tumour cells.