B-cell malignancies (B-CLL) are heterogeneous malignant diseases; they share a common cell lineage but a wide range of clinical behavior and outcomes. The most frequent chromosomal aberrations in B-CLL are deletions 13q, 11q, 17p and trisomy 12, all of which are of prognostic significance. To elucidate the genetic events perturbing important roles in the prognosis of CLL patients, FISH was performed on 20 CLL patients using LSI ATM locus specific probe mapping for 11q22.3 locus flanking the ATM region and LSI p53 locus specific probe mapping for 17p13.1 locus flanking the p53 region. The results were compared with 10 healthy volunteers as normal control. All the 10 healthy subjects (100%) showed diploid sample. 25% of patients were positive for chromosome 11q22.3 deletion,35% were positive for chromosome17p13.1 deletion while 20% were positive for combined deletions of both chromosome 11 and chromosome 17. Out of the 5 cases positive for 11q23 deletion, 4/5 had showed combined deletion while out of the 7 cases positive for 17p13.1 deletion, 4/7 showed combined deletion. A statistically significant positive correlation was found between the mean number of cells showing monoallelic LSI ATM deletion and CD5/19(estimate disease load) and CD38 (marker of disease progression and shorter patient survival), as well as a statistically significant negative correlation was found between the mean number of cells showing 11q22.3 deletion and the hemoglobin level of the studied group. CD5/19 was significantly higher in 17p13.1 deletion positive than negative cases. Patients with combined deletion had significantly higher age, CD38 and CD5 than other CLL patients without deletions. ATM gene deletion (11q 22.3 deletions) can be considered a predictor for poor prognosis in CLL patients and this opens the possibility for clinical trials aiming to treat patients with ATM gene deletion early in the course of the disease regardless their clinical stage. Patients with leukemia cells that have del (17p) have an inferior prognosis and appear resistant to standard chemotherapy regimens.