We investigated the hematological and clinical status of 145 patients with de novo myelodysplastic syndrome (MDS), 14 of whom (10%) had eosinophilia in the bone marrow (MDS-Eo). Most of these 14 patients had severe anemia. Their bone marrow cells exhibited trilineage dysplasia and some morphological abnormalities in the eosinophils, including disproportion of eosinophilic granules, basophilic granules, a ring-shaped nucleus, and vacuolation in the cytoplasm. However, these abnormalities were less prominent than those of acute myelomonocytic leukemia with eosinophilia (FAB: M4Eo). Three of the 14 MDS-Eo patients had refractory anemia (RA), seven had RA with excess of blasts (RAEB), and four had RAEB in transformation. Cytogenetic analysis revealed chromosomal abnormalities in 12 of 13 MDS-Eo patients (92%), in particular, there were major karyotypic abnormalities (MAKA) in eight patients (62%). Cytotoxic agents were not effective in the treatment of four patients after leukemic transformation occurred. These four patients died of the leukemic transformation while seven died of bone marrow failure. The other three MDS-Eo patients are still alive; two of them have already transformed to a leukemic phase. The duration of survival of these patients was significantly shorter than that of the other MDS patients. These findings suggest that bone marrow eosinophilia in MDS may be a poor prognostic factor that is strongly related to the existence of MAKA.