The diagnosis of myelodysplastic syndromes (MDS) is mostly based on clinical information, cytology, biology (particularly cytogenetics). However, bone marrow biopsy (BMB) brings useful information for the diagnosis and for the assessment of prognosis, in various entities. For ex. BMB allows to recognize hypoplastic type of MDS and MDS with fibrosis. BMB is particularly useful in the appreciation of the presence of blast cells and their quantification, allowing the diagnosis of refractory anaemia with excess of blasts and the identification of transformation in acute myeloid leukaemia, particularly in MDS with hypoplasia and MDS with fibrosis. BMB plays also an important role in differential diagnosis with diverse benign or malignant hemopathies.