The morphological features of Langerhans cell histiocytosis (histiocytosis X) are characteristic but the diagnosis can on occasion be difficult. A case is presented that illustrates the diagnostic value of immunohistochemistry in the differential diagnosis of this condition. The cells of Langerhans cell histiocytosis were found to express CD1, CD4, CD11b and CD11c. They also reacted with EBM11, UCHM1, KB61 and HLA-DR. Occasional cells showed nuclear staining with Ki67, but no other lymphoid antigens were detected. Immunoreactivity of the cells of Langerhans cell histiocytosis with antibodies that recognize antigens present on macrophages provides further evidence for immunological similarities between these cell types.