Myelodysplastic Syndrome (MDS) is a blood disorder that involves the ineffective production (or dysplasia) of myeloid cells, which become misshapen and dysfunctional. About 30% of MDS cases progress to become Acute Myelogenous Leukemia (AML). For this reason, MDS is often considered to be a preleukemic condition.

The number of people with MDS is not known because it can go undiagnosed and there is no mandated tracking of the syndrome. Some estimates are on the order of 10,000 to 20,000 new cases each year in the United States. The incidence of MDS is probably increasing as the age of the population increases, and some have suggested may be as high as 15 cases per 100,000 persons over age 70 per year.

MDS is known to be caused by exposure to benzene, ionizing radiation, and certain chemotherapy drugs. The disease has also been associated with exposure to pesticides, solvents, and cigarette smoking.

Canada and Germany have recognized MDS as an occupational disease from benzene exposure, and benzene-exposed workers who develop MDS in Canada and Germany may be compensated for the illness under the occupational disease laws of those countries.