We performed prospective sequential cytogenetic studies in 76 patients with myelodysplastic syndromes (MDS) followed up to 82 months. Their karyotypes were followed routinely, regardless of clinical status. The incidence of evolutive karyotypes was similar in patients with a normal karyotype at referral and in patients with clonal abnormalities at diagnosis (24.5 and 26.1%, respectively). We did not find association between karyotype evolution and leukemic transformation or reduced survival, since the majority of secondary cytogenetic changes in evolutive karyotypes of our patients were aberrations with good or intermediate prognosis. Therefore, we concluded that only particular cytogenetic events are related to disease progression, while others represent secondary changes of little biologic and prognostic significance.