Childhood myelodysplastic syndromes (MDSs) are clonal myeloid neoplasms characterized by impaired production of circulating hematopoietic elements (generally, hemoglobin concentration < 10 g/dL, platelet count < 100 × 109/L, or absolute neutrophil count < 1.8 × 109/L), frequently accompanied by morphologic dysplasia and recurrent genetic abnormalities and lacking features of overproduction of additional hematopoietic lineage elements. In light of its rarity in comparison to its clinicopathologic mimics, minimal diagnostic criteria have been proposed (see Table 13.1).