Chronic myelogenous leukaemia (CML) is a myeloproliferative neoplasm that is almost always characterised by the presence of t(9;22)(q34;q11.2). Approximately 5% to 10% of CML patients lack cytogenetic evidence of t(9;22)(q34;q11.2) but have the breakpoint cluster region (BCR)/ABL1 fusion, as revealed by fl uorescence in situ hybridisation (FISH) or the reverse transcription-polymerase chain reaction (RT-PCR). We present a case of Philadelphia-negative CML with a cryptic insertion of BCR at 9q34. A 22-year-old woman incidentally presented with marked leucocytosis and anaemia. Her complete blood count results were as follows: white blood cells, 238.61x10(9)/L; haemoglobin, 9.6 g/dL; platelets, 395x10(9)/L. A peripheral blood smear showed leucocytosis with neutrophilia, basophilia, left-shifted neutrophils, and circulating blasts comprising 2% of the total leucocytes. The bone marrow showed a striking increase in megakaryocytes and granulocytic precursors. The myeloid/erythroid ratio was 7.4:1, and blasts comprised up to 1.8% of all nucleated cells. Bone marrow sections revealed active megakaryopoiesis and granulopoiesis with 100% cellularity. Chromosomal analysis revealed a normal karyotype. However, interphase FISH using a dual-colour BCR/ABL1 fusion probe showed an atypical pattern consisting of one red, two green, and one fusion (1R2G1F) signal in 97.5% of the 200 analysed cells. Metaphase FISH revealed a single BCR/ABL1 fusion signal on chromosome 9. RT-PCR was positive for BCR/ABL1 (b3a2). Quantitative PCR revealed a normalised copy number of 15.32. The patient started her treatment with imatinib, reached a complete molecular response eight months afterwards, and has been coping well without any adverse events.