Myelodysplastic syndrome: One of a group of disorders characterized by abnormal development of one or more of the cell lines that are normally found in the bone marrow. Patients can develop a variety of symptoms related to anemia, low or high white blood cell count, infections, and bleeding problems. Myelodysplastic syndrome may progress and become acute leukemia. The well-recognized myelodysplastic syndromes include refractory anemia, refractory anemia with ringed sideroblasts, refractory anemia with excess blasts, refractory anemia with excess blasts in transformation, chronic myelomonocytic leukemia, chronic myelomonocytic leukemia in transformation, and unclassified myelodysplastic syndrome. Also known as preleukemia or smoldering leukemia.