Aim. To estimate a change in myelodysplasia in decitabine-treated patients with myelodysplastic syndromes (MDS). Subjects and methods. Thirteen MDS patients from a high-risk group were examined; 75 bone marrow puncture specimens and 67 bone marrow trepanobiopsy specimens from these patients were analyzed before and after decitabine treatment. Dysplastic changes in the hematopoietic cells were monitored during the treatment. Results. The dysplastic changes in the hematopoietic cells are a morphological portrayal of the ineffective hematopoiesis in patients with MDS. The study has indicated that the use of the hypomethylating agent decitabine promotes the restoration of cell differentiation to mature forms, causing hematopoiesis to be more effective. The incidence of myelodysplasias (including mixed double- and triple-lineage ones) was statistically significantly reduced by decitabine treatment, which was associated with a positive response to treatment as a whole. The count of cells with dysplastic features remained unchanged in patients with therapy resistance or further disease progression. Conclusion. Analysis of myelodysplastic manifestations in different hematopoietic lineages in patients with MDS should be based on the comprehensive dynamic assessment of cytological and histological parameters at both the primary diagnosis of the disease and different stages of treatment. With a response to decitabine therapy (as shown by the results of aspiration and trepanobiopsy), all cell lines displayed reduced myelodysplastic changes, indirectly indicating a decrease of the abnormal clone itself in high-risk MDS patients.