The myelodysplastic syndromes (MDS) are a group of clonal disorders, common especially in the elderly, characterised by cytopenias and dysfunctional blood cells. They are a cause of significant morbidity and premature mortality. The cause is not known in most cases. Predisposing factors that have been identified include cytotoxic chemotherapy, benzene and other environmental mutagens, and bone marrow transplantation. Clinically patients present with effects of deficiency of erythrocytes, neutrophils and/or platelets or the diagnosis may be made unexpectedly after routine blood testing. The bone marrow is generally hypercellular and often disorganized; abnormal in vitro cell growth is common. Non-random cytogenetic abnormalities are characteristic and helpful diagnostically; certain subtypes are associated with specific clinical and cytological features. Especially noteworthy are the 5q- and 7-syndromes. The outlook generally is poor. Death comes about from transformation to acute myeloid leukemia (AML), from the complications of cytopenias, or from intercurrent illness. Treatment is unsatisfactory except in young patients who can undergo allografting. Treatments of uncertain value include intensive or gentle chemotherapy. Of the cytokines erythropoietin and granulocyte-macrophage colony-stimulating factor (GM-CSF) so far seem the most promising. However, for the majority management is limited to provision of appropriate supportive care.