Langerhans cell histiocytosis (LCH) is a clonal, proliferative disorder of phenotypically immature CD1a(+) Langerhans cells (LC). The aetiology of LCH is unknown and data supporting an immune dysregulatory disorder as well as a clonal neoplasm have been reported. Telomere shortening has been associated with cancers and premalignant lesions as well as promoting chromosomal instability. To determine whether LCH LC have altered telomere lengths, we used dual detection of CD1a expression by immunofluorescence and telomere length by fluorescence in situ hybridization of LCH LC and lymphocytes in local, multisystem and systemic LCH and compared these with telomere lengths of LC and lymphocytes in reactive lymph nodes. LCH LC showed significantly shorter telomere lengths than LC from reactive lymph nodes or unaffected skin. Lymphocyte telomere lengths showed similar profiles among the different samples. These data show a significant telomere shortening in LCH LC in all stages of disease involvement compared with LC from reactive lymph nodes, suggesting that LCH may share mechanisms of telomere shortening and survival with clonal preneoplastic disorders and cancer, although an initiating infectious or immune event is still possible.