Forty-three patients with persistent and polyclonal B-cell lymphocytosis (PPBL) were studied. The PPBL diagnosis was based on the presence of a polyclonal lymphocytosis and the detection of binucleated lymphoid cells on peripheral blood examination. In order to define the cytogenetic profile in these patients, conventional cytogenetic analysis and fluorescence in situ hybridization were performed at diagnosis in all patients and also at follow-up in 10 patients. When excluding + i(3q) and PCC, chromosomal instability is a common occurrence in PPBL and is characterized by other independent clonal abnormalities, del(6q), + der(8) or + 8, polyploid karyotype, structural changes, aneuploidy and/or non clonal chromosomal aberrations with either loss or more frequently gain of chromosomes. These data show the presence of a chromosomal instability in 67.5% of PPBL patients. Finally, ATR amplification was detected by hybridization with BAC probe 26217 in + i(3q) positive metaphase cells. No ATR deletion was observed in the + i(3q) negative B-cells. As the natural history of PPBL remains unclear, it is necessary to diagnose PPBL patients and useful to recommend a careful and continued long follow-up in all PPBL patients.