Common variable immunodeficiency (CVID) results in defective B cell differentiation and impaired antibody production and is the most common symptomatic primary immunodeficiency. Our aim was to evaluate the correlation among B cell subgroups, kappa-deleting recombination excision circle (KREC) copy numbers, and clinical and immunological data of the patients with CVID, and evaluate the patients according to classifications currently available to define the role of KREC copy numbers in the diagnosis of CVID. KREC analysis was performed using a quantitative real-time polymerase chain reaction assay, and B cell subgroups were measured by flow cytometry. The median age of the patients (n = 30) was 25 (6-69) years. Parental consanguinity ratio was 33%. The median age at diagnosis was 15 (4-59), and follow-up period was 6 (1-37) years. CD19(+) and CD4(+) cell counts at the time of diagnosis were low in 66.7% and 46.7% of the patients, respectively. CD19(+) cell counts were positively correlated with KREC copy numbers in patients and healthy controls. CD19(+) cell counts and KREC copy numbers were significantly reduced in CVID patients compared to healthy controls as expected. KRECs are quantitative markers for B cell defects. We found low CD4(+) cell numbers, recent thymic emigrants, and lymphopenia in some of the patients at diagnosis, which reminds the heterogeneity of CVID's etiology. In this study, a positive correlation was shown between CD19(+) cell counts and KREC copy numbers. Low KREC copy numbers indicated B cell deficiency; however, high KREC copy numbers were not sufficient to rule out CVID.