Rearrangements of 6p are frequent in both myeloid and lymphoid malignant hematological disorders. High-mobility group AT-hook 2 ( HMGA2) rearrangements have been described in myelofibrosis with myeloid metaplasia (MMM) and also in myelodysplasia. High-mobility group A proteins are nonhistone nuclear proteins that bind DNA and regulate the transcriptional activity of many genes. We used FISH, with bacterial artificial chromosome RP11-513I15 probe, to study 16 cases of myeloid malignancies with chromosome 6 short arm rearrangements, most of them following myeloproliferative disorders. Among these we found two 6p21.3 duplications and one 6p21.3 triplication involving HMGA1 in four cases of myelodysplasia with and without myelofibrosis. In these four cases, duplications and triplication were partially masked at the cytogenetic level by a derivative chromosome 6 resulting from translocation with another chromosome. HMGA1 proteins have been recently found overexpressed in human leukemias, but to our knowledge this is the first reported duplication of HMGA1.