Abstract Deletions of the long arm of chromosome 5 [del(5q)] are frequent chromosome aberrations with known prognosis in myelodysplastic syndromes and acute myeloid leukemia (AML). However, in chronic lymphocytic leukemia (CLL), they are rare and have been reported only as karyotypic results without known prognosis. In the present study, we report a novel conventional and molecular cytogenetic study of two CLL patients carrying interstitial del(5q) in order to contribute to the identification of rare recurrent aberrations and their prognostic impact in CLL. Karyotypic and fluorescence in situ hybridization analysis that used probes for the most common aberrations of CLL demonstrated that del(5q) was the sole chromosome abnormality in both patients at the time of diagnosis. None of these patients had a history of exposure to chemotherapy or radiotherapy. Both patients had disease that was still staged as Binet A at 28 and 18 months after diagnosis, respectively, without receiving any therapy because of their good clinical condition. Therefore, it could be suggested that del(5q) may not be associated with an adverse prognosis in CLL and is not related with therapy-induced chromosome changes. Further studies are required to elucidate the prognostic value of these deletions in more CLL patients, which could be advisable for prognostic and therapeutic purposes, as well as to identify candidate genes that may potentially play a role in the pathogenesis of CLL.