Although the association of malignancies and systemic mast cell disease (SMCD) is well established, the nature of this relationship is poorly understood. The observation of 19 malignancies in 17 of 60 patients with SMCD raised several questions regarding the chronological relationship of onset of SMCD and the malignancies, whether these patients are at increased risk for developing malignancy, and whether the distribution of solid vs hematologic malignancies indicates a relationship between SMCD and a particular tumor. The following malignancies were observed: eight solid tumors, seven acute nonlymphocytic leukemias, three malignant lymphomas, and one refractory anemia with excess blasts in transformation. The majority (13/17) of patients were found to have malignancies before, or within 12 months of, SMCD diagnosis. Statistical analysis suggested that patients with SMCD are not at increased risk for malignancies subsequent to the diagnosis of SMCD. The varied types of solid malignancies observed indicated a random distribution, in contrast to the hematologic malignancies that appeared to primarily affect the myeloid cells.