Background: Chronic lymphocytic leukemia (CLL) is a B lymphocyte malignancy that affect mainly adults, its clinical course is variable. 13q deletion is a common cytogenetic abnormality found in CLL patients and in general, it is associated with a good prognosis and a benign clinical course. However, it was noticed that there are clinical heterogeneity among CLL patients harboring 13q deletion which may be attributed to type, size, or rate of 13q deletion. So, it was important to clarify the source of this heterogeneity. Methods: Seventy one CLL patients were examined by conventional cytogenetic and fluorescent in situ hybridization (FISH) for 13q14 deletion. Results: Deletion of 13q14 was detected in 45/71 CLL patients (63.4%) and those had significantly lower absolute lymphocytic count and hemoglobin concentration, 53.3% of positive patients were in Binet stage A, and 31.1% of them required chemotherapy. In 38 patients 13q14 deletion was found as an isolated abnormality and those had significantly lower WBCs count, absolute lymphocyte count and LDH than those with accompanied cytogentic abnormalities (non-isolated). The deletion was found at a rate (≥80%) in (8/ 38) patients with isolated deletion type and those had a significantly shorter time to treatment. The 13q14 deletion was biallelic in only 3 patients out of 38 with isolated type and all of them had a diffuse bone marrow infiltration pattern. Conclusions: 13q14 deletion is a common cytogenetic abnormality in CLL patients and has a variable impact on the disease condition according to specific features of deletion such as isolated or non- isolated, deletion rate, and type of deletion being monoallelic or biallelic.