Isodicentric chromosome 21, idic(21)(p11.2), is a rare but recurrent cytogenetic aberration in acute lymphoblastic leukemia. We describe here a novel case of acute myeloid leukemia (AML) with double idic(21)(p11.2). A 35-year-old man was diagnosed as having de novo AML with multilineage dysplasia because of 30% myeloperoxidase-positive blasts and trilineage dysplasia in the bone marrow. Surface marker analysis revealed that the blasts were positive for CD7, CD13, CD33, CD34, and HLA-DR. Chromosome analysis and spectral karyotyping showed 47,XY,+21,idic(21)(p11.2)x2, leading to pentasomy 21q. Fluorescence in situ hybridization demonstrated two RUNX1 signals on the idic(21)(p11.2), resulting in a total of five RUNX1 signals in metaphase spreads and interphase nuclei. These results suggest that the idic(21)(p11.2) could be implicated also in the pathogenesis of AML through amplification of genes including RUNX1 located on 21q.