Langerhans cells histiocytosis (LCH) is a proliferative disorder of Langerhans cells. The lesions are normally characterized by infiltration of eosinophils, neutrophils, lymphocytes, plasma cells, and Langerhans cells. The specific cells of LCH contain Birbeck granules, express the phenotype of Langerhans cells but with markers fixed at an early stage of activation, and are functionally defective in antigen-presenting ability. The disease most often affects children; when it occurs in older patients, anal and groin involvement is quite common and vulvar lesions can be found in older females. The authors report a case of a 64-year-old woman with LCH of the vulva and diabetes insipidus. An immunohistochemical and ultrastructural study of the vulvar lesions showed an infiltrate in which antigenically and morphologically mature Langerhans cells, monocytoid cells, and cells with an intermediate phenotype between monocytes and Langerhans cells were concurrently observed. Although the clinical and histological aspects of LCH are well established, the pathogenetic mechanism of lesions is not yet known. The finding of an infiltrate composed by Langerhans cells and many putative precursors of these cells suggests the hypothesis of an in situ differentiation of Langerhans cells from immature monocytoid precursors.