Twenty-eight patients with histiocytosis X (HX) who had bone marrow aspirates and/or trephine biopsies taken at diagnosis were studied. Five of the 28 patients (18%) had bone marrow specimens with clusters of benign histiocytes characteristic of histiocytosis X. Several statistically significant clinical and laboratory differences were found between those patients with positive marrows versus those with uninvolved bone marrows. Presence of HX in the bone marrow was more frequently associated with fever (P less than 0.01), splenomegaly (P less than 0.01), rash (P = 0.04), diabetes insipidus (P = 0.09), and platelet counts less than 175 X 10(9)/L(P = 0.01). Two of the five patients with bone marrow involvement died, whereas there were no deaths due to HX in the other 23 patients. From this review, it is suggested that bone marrow involvement at diagnosis may be associated with more extensive and potentially fatal disease. HX of the marrow was more easily diagnosed from trephine biopsy sections than aspiration smears.