Serial cytogenetic studies were performed in 33 patients with myelodysplastic syndrome in order to establish the frequency of karyotypic evolution and to correlate the chromosome and clinical findings during the course of the disease. Fifteen of the 33 patients (45%) showed abnormalities in the first cytogenetic study and this percentage increased to 57% during the course of the disease. A stable karyotype (normal or abnormal) was found in 19 patients (58%), whereas the rest (42%) showed an unstable karyotype. Trisomy 8, monosomy 7, and del5q were the most frequent abnormalities, not only at presentation, but also during karyotypic evolution. Seven patients (23%) with a known evolution proceeded to leukemia; four of them had stable (22%) and three unstable (25%) karyotypes; however, 33% of patients with unstable karyotypes and only 5% with stable karyotypes died from complications of the disease. Our results suggest that karyotypic evolution is relatively frequent in these patients; this evolution could be related to a poor clinical prognosis, either evolving to leukemia or death.