The immunophenotype of 6 cases of Langerhans cell histiocytosis (LCH) of the hypothalamus and 3 cases of cranial bone manifestation of LCH was investigated by means of immunohistochemistry on paraffin sections. Antibodies against S 100 protein, lysozyme, CD68 (PG-M1), CD68 (KP1), HLA-DR, beta 2 microglobulin, placental alkaline phosphatase (PLAP), the monoclonal antibody MAC 387, and a monoclonal antibody against CD1a were used. All examined cases showed positive staining of lesional cells for S 100 protein, HLA-DR, beta 2 microglobulin, macrophage associated markers and CD1a. According to the "confidence levels" of the Writing Group of the Histiocyte Society [Chu et al. 1987], a "definite diagnosis" of LCH requires the demonstration either of Birbeck granules in lesional cells by electron microscopy, or of CD1a antigenic determinants on the surface of lesional cells. Since electron microscopy of these rare CNS lesions is not possible in many cases, we are now able to give a definite diagnosis of LCH of the hypothalamus by means of immunohistochemistry for CD1 a on routinely fixed and processed tissue.