Myelodysplastic syndrome (MDS) = heterogeneous group of clonal disorders of hematopoiesis. Characterized by: hyper- or hypocellular bone marrow; abnormal cell morphology; abnormal maturation; peripheral blood cytopenias; ineffective blood cell production. All three myeloid cell lineages (erythrocytic, granulocytic, and megakaryocytic) may be involved. Considered a premalignant condition as some patients progress to AML. May present with: anemia, thrombocytopenia, neutropenia. Diagnostic approach includes: complete blood count with differential, peripheral blood smear, bone marrow studies. Treatment: supportive, including transfusions; possible bone marrow stimulation; cytotoxic chemotherapy; bone marrow transplantation has a limited role. May be caused by exposure to: cytotoxic chemotherapy; radiation; viral infection; genotoxic chemicals (e.g., benzene). May also be no known exposure. Cytogenetics include: normal karyotype; balanced chromosomal abnormality and generation of fusion oncogenes; complex karyotypes. Primarily affects the elderly. French-American-British classification: refractory anemia (RA); RA with ringed sideroblasts (RARS); RA with excess blasts (RAEB; 6-20% myeloblasts); RAEB in transition to AML (RAEB-T; 21-30% myeloblasts); chronic myelomonocytic leukemia. 2008 WHO classification: refractory cytopenia with unilineage dysplasia; RARS; RAEB; refractory cytopenia with multilineage dysplasia; MDS with isolated deletion of 5q; MDS, unclassifiable; childhood MDS