Langerhans' cell histiocytosis (LCH) is characterized by an accumulation and/or proliferation of cells with a Langerhans' cell (LC) phenotype. The aetiology and pathogenesis of LCH are unknown; it is suggested that LCH is caused by an immunological dysregulation. Production of cytokines is a central feature of immunological regulation. LCH lesions and normal LCs were studied for the presence of cytokines known to influence the functioning of LCs: IL-1 alpha, IL-1 beta, IL-4, GM-CSF, IFN-gamma, TGF-alpha, TGF-beta, bFGF, and TNF-alpha. Cytokines were abundantly present within LCH lesions; LCH cells stained for IL-1 alpha, IL-1 beta, IL-4, GM-CSF, TGF-alpha, TGF-beta, TNF-alpha, and IFN-gamma. Macrophages, lymphocytes, eosinophil granulocytes, and, surprisingly, multinucleated giant cells were also sources of cytokines. These results suggest that cytokines play a prominent role in the pathogenesis of LCH and may explain phenomena that often occur in LCH, such as osteolysis and fibrosis and the recruitment of typical inflammatory infiltrates. The results also suggest that a 'down-regulatory' signal is lacking in LCH, resulting in an accumulation and/or proliferation of abnormal LCs.