A clinical and hematopathologic review of 66 patients with systemic mast cell disease (SMCD) was undertaken to investigate the frequency and the clinical significance of associated hematologic disorders. Twenty-two patients were found to have a second hematologic disorder, 19 of which involved the myeloid cells (ten dysmyelopoietic syndromes, five myeloproliferative disorders, three acute nonlymphocytic leukemias, and one chronic neutropenia), and three of which involved the lymphoid cells (three malignant lymphomas). A chromosome analysis of the bone marrow revealed abnormalities characteristic of neoplastic myeloid disorders in four patients. Five-year survival for patients with hematologic disorders was 28% compared with 61% for other SMCD patients (P = 0.004). Patients with hematologic disorders differed significantly from other SMCD patients in that they were about 7 years older (P = 0.039), and they presented more commonly with anemia (P less than 0.001) and constitutional symptoms (P = 0.007). These patients also had less frequent skin symptoms (P = 0.003) and urticaria pigmentosa (P = 0.018). By definition, patients with hematologic disorders had a greater percent of hematopoiesis (P less than 0.001) and decreased fat cells (P = 0.011) on bone marrow biopsies. A multivariate model demonstrated that the following independent variables were associated with the presence of hematologic disorders: low hemoglobin (P = 0.001), the absence of hepatomegaly (P = 0.016), high leukocyte count (P = 0.021), and the presence of pathologic fractures (P = 0.051). The frequent coexistence of SMCD with dysplastic and neoplastic disorders of myeloid cells is consistent with the concept that SMCD itself is a disorder of myeloid cells and that the mast cell may be myeloid in origin.