Myelodysplastic syndromes are clonal haemopathies known to exist in elderly people where they are classified into 5 categories: refractory anaemia, acquired idiopathic sideroblastic anaemia, refractory anaemia with excess of blasts, refractory anaemia with excess of blasts undergoing acute transformation, and chronic myelomonocytic leukaemia. Transformation into acute leukaemia is frequent. These syndromes seem to be rarer in children, but they are often misdiagnosed. Some of their forms are particular to childhood. They include association with constitutional blood diseases predisposing to acute leukaemia, or with congenital or non-congenital malformative syndromes; there are hypoplastic forms, forms evolving towards stabilization of haematological abnormalities, and border forms with certain myeloproliferative syndromes. The diagnosis of myelodysplasia rests on morphological, functional, biochemical, isotopic data reflecting inefficient haematopoiesis, and on cytogenetic data. Studies of clonality and malignancy markers will in the near future enable us to distinguish between true preleukaemic states and polyclonal constitutional or virus-induced myelodysplasias, probably more frequent in children. The choice of treatment depends on the severity of the myelodysplastic syndrome, defined by the initial partial blastosis, the presence of cytogenetic anomaly and the certainty of clonality. In cases with poor prognosis, early bone marrow allograft is the only possible treatment.