Myelodysplastic syndromes (MDS) for a long time were an ill-defined group of disorders, the true nature of which was largely unknown. Because some patients developed acute leukemia, MDS was considered to be potentially premalignant. Cytogenetic investigations in the early 1970s brought the first clear evidence that these disorders were clonal. Further research has shown that MDS encompasses a number of cytogenetic entities, some of which are associated with clinically distinct disorders.