Langerhans cell histiocytosis (LCH) is a class I histiocytosis characterized by the presence of the pathologic Langerhans cell, an unique histiocyte. In contrast to LCH, class II histiocytosis is characterized by the proliferation of mononuclear phagocytes other than Langerhans cells and includes sinus histiocytosis with massive lymphadenopathy, viral-associated hemophagocytic syndrome, and familial hemophagocytic lymphohistiocytosis. Until now, these two classes have been considered separate, if related, entities. We report a 10-month-old girl who presented with pyrexia, hepatosplenomegaly, an eczematous skin rash, anemia, thrombocytopenia, and a markedly elevated serum IgG and IgM antibody level to cytomegalovirus. Histologic proof of both hemophagocytosis in the liver and bone marrow and LCH in the skin was obtained at presentation. The clinical course and response to treatment over 6.5 years is recorded. Although the etiology of both class I and class II histiocytosis remains unknown, we speculate that the monocytic/macrophage disorder, as well as the LCH, were both triggered by virus or viral-related monokines secreted by activated macrophages.