The distinction between RAEB, RAEB-T and AML M6a is difficult when erythroblasts in the bone marrow (BM) exceed 50%. We analyzed 19 children (2 RAEB, 13 RAEB-T and 4 AML M6a) enrolled in a prospective pathological central review in Japan and divided them into two groups according to the myeloblasts percentage among non-erythroid cells in BM: group A (n = 8), 5-19% myeloblasts; group B (n = 11), 20% or more myeloblasts. Their characteristics were very similar except for the number of myeloblasts. The median WBC was in the range of 1.0-5.0 x 10(9) L(-1), the median Hb was around 7.5 g/dL, the median MCV was greater than 90 fL and both group had Auer rods at 60-65%. Severe multilineage dysplasia was observed in most of the patients in two groups. Six with group A and seven with group B treated with AML type chemotherapy achieved complete remission. Five with group A and seven with group B undergoing SCT are alive at a median of 3 years after diagnosis. Erythroblast-rich RAEB and AML M6a in children have similar characteristics and may belong to a single disease entity.