['To study the immunoexpression of proteins related to the mitotic checkpoint (cell division cycle 20 (CDC20), mitotic arrest deficient 2 (MAD2)) and the mitotic spindle (Aurora-B) in patients with myelodysplastic syndrome (MDS).', 'Protein expression was analysed in bone marrow tissue samples from 40 patients with MDS using immunohistochemistry. Prognostic markers (transfusion dependency, depth of cytopenias, chromosomal abnormalities and survival) were also studied.', 'Higher MAD2 expression was observed among patients with platelets <50×10(9)/L than among patients with platelets ≥50×10(9)/L (42.6±22.8% vs 22.7±19.1%, respectively). Higher CDC20 expression was identified among patients with three dysplasias compared with patients who presented with one or two dysplasias (33.9±24.1% vs 10.5±5.7% vs 12.8±7.8%, respectively), among patients who exhibited a complex versus non-complex karyotype (50.0±30.2% vs 18.4±14%, respectively) and among patients with platelets <50×10(9)/L vs platelets ≥50×10(9)/L (38.2±26.2% vs 16.1±12.4%, respectively). Higher Aurora-B expression was found in patients with an abnormal versus normal karyotype (21.2±13.2% vs 7.5±5.0%, respectively). High expression of MAD2 and CDC20 (≥50%) was associated with severe thrombocytopenia. We also found statistically significant differences in the overall survival rate when comparing different degrees of CDC20, MAD2 and Aurora-B protein expression.'] Are proteins of the mitotic checkpoint and spindle related to chromosomal instability and unfavourable prognosis in patients with myelodysplastic syndrome?