The myelodysplastic syndromes (MDSs) are a group of haematological disorders found predominantly in the elderly, and are defined by morphological abnormalities of the three cell lines. None of the abnormalities is specific for MDS but, when combined with active or hyperactive haemopoiesis and refractory cytopenia in an elderly person, they suggest the diagnosis. The French-American-British co-operative group divided MDSs into five types: refractory anaemia; refractory anaemia with ring sideroblasts; refractory anaemia with excess of blasts; refractory anaemia with excess of blasts in transformation; and chronic myelomonocytic leukaemia. The classification was based on the number of blasts in the bone marrow, dysplastic features in one or more cell lines, ringed sideroblasts, blasts in the peripheral blood, Auer rods in the granulocyte precursors and absolute monocytosis. MDS can be a primary condition and of unknown aetiology or develop secondary to the action of a known agent such as an alkylating agent, chemical or recombinant human granulocyte colony-stimulating factor. The pathogenesis of MDS is thought to be a multi-step process which begins with a somatic mutation in the pluripotential stem cell, is irreversibly altered and acquires a survival advantage. The abnormal clone expands at the expense of normal haemopoiesis and undergoes further genetic change to give a progressively more malignant phenotype. Present theories of the development of MDS are speculative but further research could cast light on the development of other haematological disorders.