Histiocytosis X and mastocytosis are proliferative processes that may have similar cutaneous manifestations. However, a positive Darier's sign (urtication on stroking of the lesion) is thought to reliably distinguish between these two diseases. We recently studied a 13-year-old girl with a 2-year history of extensive skin lesions and a positive Darier's sign. Routine histopathologic studies revealed a polymorphous cutaneous infiltrate composed of histiocytes, mast cells, eosinophils, and lymphoid cells. Electron microscopic studies demonstrated Langerhans granules in some of the histiocytes, and immunologic studies of frozen tissue showed that a significant subpopulation of the histiocytes marked as Langerhans cells. Giemsa staining of specimens from eight other cases of cutaneous histiocytosis X from our files revealed mast cells in all of the lesions, although none showed the abundance of mast cells present in the case with urtication. Our studies emphasize the often polymorphous nature of the cell population in cutaneous histiocytosis X and demonstrate that confusing clinical findings can result when the mast cell population in histiocytosis X produces urtication.