IL-17A is a T cell-specific cytokine(1) that is involved in chronic inflammations, such as Mycobacterium infection(2), Crohn's disease(3), rheumatoid arthritis(4) and multiple sclerosis(5). Mouse models have explained the molecular basis of IL-17A production(6,7) and have shown that IL-17A has a positive effect not only on granuloma formation(8) and neurodegeneration(9) through unknown mechanisms, but also on bone resorption through Receptor activator of NF-kappa B ligand (RANKL) induction in osteoblasts(4,10). Langerhans cell histiocytosis (LCH) is a rare disease of unknown etiology, lacking an animal model, that cumulates symptoms that are found separately in various IL-17A-related diseases, such as aggressive chronic granuloma formation, bone resorption and soft tissue lesions with occasional neurodegeneration(11,12). We examined IL-17A in the context of LCH and found that there were high serum levels of IL-17A during active LCH and unexpected IL-17A synthesis by dendritic cells (DCs), the major cell type in LCH lesions. We also found an IL-17A-dependent pathway for DC fusion, which was highly potentiated by IFN-gamma and led to giant cells expressing three major tissue-destructive enzymes: tartrate resistant acidic phosphatase and matrix metalloproteinases 9 and 12. IFN-gamma expression has been previously documented in LCH13 and observed in IL-17A-related diseases(14-17). Notably, serum IL-17A-dependent fusion activity correlates with LCH activity. Thus, IL-17A and IL-17A-stimulated DCs represent targets that may have clinical value in the treatment of LCH and other IL-17A-related inflammatory disorders.