Clinical, hematologic, and cytogenetic data of nine patients with refractory anemia with excess of blasts in transformation (RAEB-t), classified according to the French-American-British Cooperative Group for myelodysplastic syndrome (MDS), are reported. At diagnosis, eight out of nine cases, had chromosomal abnormalities and three out of nine developed acute leukemia. Karyotype studies allowed individualization of two groups of patients: five with nonrandom major karyotype abnormalities (MAKA) including hypodiploidy, chromosomes 5 and 7 involvement, at least four other abnormalities, and a poor prognosis (survival always under 3.5 months); and four patients with either normal karyotypes or minor karyotype abnormalities (MIKA) (no more than three abnormalities) and a better prognosis (survival from 14 to 38 months). Karyotype appears to be a major prognostic factor among RAEB-t.