Acute promyelocytic leukaemia (APL) is characterised by proliferation of abnormal promyelocytes. The reciprocal translocation between the long arms of chromosomes 15 and 17, and the fusion between the retinoic acid receptor (RARa) gene, and PML gene, is unique to APL. Because of unsuccessful cytogenetic analysis of conventional G-banding technique (mitoses were not observed), we diagnosed three non-treatment patients with APL by following molecular methods: reverse transcription ? polymerase chain reaction (RT-PCR), fluorescence in situ hybridization (FISH) and comparative genomic hybridization (CGH). At the time of diagnosis our patients showed reciprocal translocation t(15;17)(q22;q12) in all cases studied (66-85% of positive bone marrow cells). With the use of CGH we observed the unbalanced chromosomal aberrations: losses of 5q13.1, 5q31.3, 9p21 regions, gain of 5q32 region and trisomy of 18 chromosome.