Serial haematopathological and cytogenetic studies disclosed three distinct clinical phase in a case of refractory anaemia (RA), a subtype of myelodysplastic syndrome (MDS; FAB group, 1982): first, chronic MDS phase (1 year 10 months) with karyotypic abnormality (45, XY, --7) (Clone I); second, hypo-aplastic phase concurrent with first clonal evolution (45, XY, --7, 12p--) (Clone II); third, acute myelomonocytic leukaemia phase (6 months) with second clonal evolution (45, XY, --7,t (1q --; Bq+), Bq --, 12p --) (Clone III). In the second phase the bone marrow became almost aplastic as Clone II expanded progressively, indicating simultaneous occurrence in Clone II stem cells of growth advantage for self-renewal function over Clone I and normal stem cells, and arrest of differentiation. These observations support the hypothesis that leukaemic change in MDS, at least in RA, occurs by stepwise clonal evolution(s), not by progressive arrest of differentiation in original MDS clone.