We performed a retrospective study of 83 patients with myelodysplastic syndrome (MDS) to clarify the clinical and cytogenetic implications of disease evolution. Twenty-three patients showed disease progression; six of the 11 patients whose disease evolved within 100 days showed complex cytogenetic aberrations and most of them died within 300 days. Of the patients who survived more than 300 days, those with high bone marrow (BM) blast percentages experienced significant disease progression, but we noted no cytogenetic indicators for disease evolution at the later phase. Sixty percent of patients showing karyotypic evolution without disease evolution had deletion-type chromosome changes. The most frequent anomaly in patients with disease evolution who survived more than 300 days was an additional numerical change, whereas patients with disease evolution who survived less than 300 days showed karyotypic instability. It was difficult to predict disease progression for patients whose disease evolved more than 300 days after diagnosis, but in some patients the presence of additional numerical changes was related to disease progression. The cutoff level of early disease evolution was 100 days after diagnosis, and most patients with complex abnormalities survived less than 300 days with or without disease evolution.