Over an 18-year period, 10 myelodysplastic syndrome (MDS) patients with complex hyperdiploid karyotypes were identified. According to the FAB classification, the 10 patients were subclassified as three refractory anemias (RA), three refractory anemias with excess blasts (RAEB), two RAEB in transformation (RAEB-t), and two unclassified MDS. According to the WHO classification, the diagnoses were two RA, one refractory cytopenia with multilineage dysplasia, two RAEB-1, one RAEB-2, two unclassified MDS, and two acute myeloid leukemia. Six were secondary MDS. Four patients showed marked dyserythropoiesis; three of these were secondary MDS. The chromosome number ranged from 47 to 62, and clonal evolution or composite karyotypes were noted in 7 patients. Seven patients had at least one clone with >50 chromosomes. Recurrent defects included chromosome 5, 17, and 13 abnormalities. Notably, trisomy 8 and monosomy 7 were rare in that group of patients. Three of four patients with marked dyserythropoiesis shared abnormalities of both chromosomes 13 and 17.