Histiocytosis X is a complex and poorly understood entity. Nevertheless, it would appear as if certain themes are found recurrently throughout the literature dealing with this disease and a review of them serves as a useful summary. 1. Problems with Nomenclature. To name or categorize a disease based on end-organ pathology is generally not clinically useful, but this is what we have done with histiocytosis X. It has caused substantial confusion among physicians and patients alike concerning diagnosis, prognosis, and treatment. Further attempts at improving the nosology of this disease will not be useful unless those new names also reflect scientific advances in our understanding of etiology, pathogenesis, and therapy. 2. Identification of the Langerhans' Cell as the Consistent Pathognomonic Cell in the Lesions of Histiocytosis X. Although the Langerhans' cell was identified more than a century ago, it has only recently been recognized as the cell that proliferates in this disease. Nevertheless, several important questions remain regarding the relationship of the Langerhans' cell to histiocytosis. Foremost among these questions is whether the Langerhans' cell is a truly normal Langerhans' cell, responding appropriately to immune system signals, or if it is an abnormal variant, possibly even neoplastic. 3. Recognition that Immune System Dysfunction Is a Critical Part of Histiocytosis X. The immune system is the focus of most recent clinical research. Results of these studies are obviously important with regard to both the biology and management of this disease. 4. Histiocytosis X Is an Extremely Heterogeneous Clinical Disorder. As mentioned before, the term histiocytosis X was originally intended by Lichtenstein to describe a pathologic, and not clinical, entity. It is rare to find two patients with this disease who are exactly alike. To make matters even more confusing, the disease includes both infants with disseminated fatal disease as well as middle-aged adults with solitary bony lesions. 5. The Disease Requires Improved Therapy, but it Is a Difficult Setting in which to Perform Clinical Studies. Improved therapy is required in patients with this disease, especially those with the disseminated form. But it will be difficult to develop improved therapy until definitive answers are provided to some of the basic questions of etiology and pathogenesis. Unfortunately, these clinical studies are not readily available because of the rare occurrence of this disease and its extreme clinical heterogeneity.(ABSTRACT TRUNCATED AT 400 WORDS)