Chronic lymphocytic leukemia (CLL) is one of the most common hematologic malignancies in adults. Although the molecular pathogenesis of CLL is poorly defined, both amplification of oncogenes or inactivation or loss of tumor suppressor gene have been described. 13q14.3 deletions are the commonest chromosomal aberrations associated with B-CLL.To better characterize these 13q14.3 deletion, to mark out the region as a first step toward the identification of the putative tumor suppressor gene involved in CLL, and to study the relation between these deletions and other prognostic clinical and laboratory markers, we performed this study using Fluorescence In Situ Hybridization (FISH) technique by chromosome 13 unique sequence probe applied to the interphase cells of 1 4 newly diagnosed cases of B-CLL.These cases were classified according to lymphocyte morphology and/ or surface markers CD5, CD19 and FMC7 into 2 groups. Group1, typical B-CLL case with typical lymphocyte morphology and surface markers CD5+, CD19+, FMC7- and group 2 ,atypical B-CLL cases, with either atypical lymphocyte morphology or atypical surface markers: CD5+, CD19+, FMC7+ or CD5-, CD19+, FMC7+.13q14.3 deletion were found in 8/14 (57.1%) and its incidence was significantly higher in the typical group of cases which has a better prognosis than atypical group.These findings suggest that chromosome 13q14.3 harbors a tumor suppressor gene whose loss contribute to the development of CLL and that this molecular aberration is a good prognostic marker in CLL.