Chronic lymphocytic leukaemia (CLL) with increased prolymphocytes (CLL/PL) has been defined by the World Health Organization (WHO) classification and considered as a progressive and clinically aggressive variant of CLL. To further characterize the biological features of this disease, we performed IgV H gene mutational status, FISH and highresolution comparative genomic hybridization (HR-CGH) analysis in 17 cases of CLL/PL. All CLL/PL utilized members of V H 1, V H 3 and V H 4 families, with the highest prevalence of the V H 1-69 gene. In all but one cases analyzed, the V H genes were unmutated. The FISH and HR-CGH analyses showed frequent occurrence of trisomy 12, del(11)(q23), del(17)(p13) and genetic imbalances, but recurrent genetic lesion characteristic for CLL/PL was not found. The follow-up HR-CGH analysis of two cases showed that increase of prolymphocytes in the course of CLL or CLL/PL are associated with clonal evolution and selection of the tumour clone. In conclusion, this study suggests that CLL/PL is a relatively homogeneous disease regarding V H gene mutation, but heterogeneous regarding genetic lesions. The heterogeneity and high number of genomic imbalances found in CLL/PL suggest that a genome-wide instability of the neoplastic cells may play a role in the development of the disease.