Langerhans cell histiocytosis (LCH) is characterized by the proliferation of clonal dendritic cells, while hemophagocytic lymphohistiocytosis (HLH) is an extreme inflammatory process sustained by the uncontrolled activation of macrophages. HLH can be primary or secondary, the latter arising in infectious, autoimmune or neoplastic disorders. We hereby present a young girl who developed secondary HLH while being treated for relapsed multisystem LCH under the LCH III Protocol. She fulfilled 5 of 8 HLH-2004 criteria (fever, splenomegaly, pancytopenia, ferritin level >500 μ/l and sIL-2R >2400 IU/ml) and was successfully treated by the HLH-2004 Protocol for secondary HLH. She remains in good health, apart from insipid diabetes she developed as a complication of LCH. Considering that the occurrence of HLH in LCH patients has been reported before, the case history presented here yields additional support for the hypothesis that the pathogenesis of the two histiocytoses--LCH and HLH--may indeed overlap to a considerable extent.