Objective To investigate clinical and experimental features of APL with tetraploid clone characterized by double t (15 ; 17). MethodsFive cases of APL with tetraploid clone characterized by double t(15;17) were chosen. Cytogenetic examination of bone marrow was performed with bone marrow or short-period culture. R banding technique was used for karyotype analysis. DNA content in one case was determined by flow cytometry. Immunophenotyping was performed by using a panel of monoclonal antibodies :CD2, CD13, CD15, CD33 and CD34. PML/RARα fusion gene was detected by interphase FISH using dualcolor PML/RARα probe in one case and by RT-PCR in two cases. ResultsAll cases were male with a median age of 38. Their marrow cell morphology examination showed marked hyperplasia with large leukemic cells that had bizarre nuclear configuration. Chromosome analysis revealed that a leukemia clone with tetraploid or near-tetraploid karyotype characterized by double t(15;17) (q22 ;q12) in five cases, of which, one also had a diploid clone with t(15;17) and a normal cell;two had some cells with normal karyotypes.PML/RARα fusion gene was detected by FISH in one of 5 cases and by RT-PCR in 2 of 5 cases. CD33 expression was found in one case. CD13 and CD33 expressions were seen in the other four cases, of which,CD34 or CD2 co-expression was found in one case and in two cases respectively. The result of DNA content showed a single peak which indicated only tetraploid clone whose DNA index was 1. 998 with CV of 8. 2%.All patients obtained complete remission after the treatment with ATRA and/or arsenic trioxide. Conclusions These results indicate that API, patients with tetraploidy and near-tetraploidy have giant and bizarre blasts.Most patients have short-type PML/RARα transcripts. The tetraploidy in APL does not appear to affect the response to treatment of ATRA.