A 2-year-old girl with Down syndrome (DS) developed acute megakaryoblastic leukemia (AMKL) following a transient myeloproliferative disorder (TMD). The blast cells showed an altered karyotype of 47,XX,r(7),+21c. Serial cytogenetic studies during the course of the illness showed rapid stepwise clonal chromosome changes, including a ring chromosome 7, associated with treatment refractoriness. We reviewed 10 published cases of Down syndrome-related AMKL (DS-AMKL) showing chromosome 7 abnormalities and found that these changes do not carry the same prognostic weight as for non-DS children. For DS-AMKL, therefore, other prognostic factors besides clonal cytogenetic changes need to be identified for planning optimal therapy.