Compared with leukemia, in which one type of blood cell is produced in large numbers, any and sometimes all types of blood cells are affected in MDS. At least 10,000 new cases occur annually in the United States.

Myelodysplastic syndrome is mainly diagnosed in people over 50, but it can affect patients as young as 2 years of age. In people over 55, the disease is twice as likely to be diagnosed in men. In younger patients the numbers of men and women are about equal. Each year, approximately two out of every 100,000 people age 60 and older are diagnosed with MDS.

Up to one third of patients diagnosed with MDS go on to develop acute myelogenous leukemia. For this reason the disease is sometimes referred to as preleukemia.

Myelodysplastic syndrome is sometimes also called myelodysplasia dysmyelopoiesis or oligoblastic leukemia. MDS is also referred to as smoldering leukemia when high numbers of blast cells remain in the marrow.

Myelodysplastic syndrome results, like leukemia, from a genetic injury to the DNA of a single cell in the bone marrow. Certain abnormalities in chromosomes are present in MDS patients. These abnormalities are called translocations, which occur when a part of one chromosome breaks off and becomes attached to a broken part of a different chromosome. The same defects are frequently found in acute myeloid leukemia.