The immunoglobulin heavy chain gene (IgHV) mutation status correlates with the clinical outcome of patients with chronic lymphocytic leukemia (CLL) treated with chemoimmunotherapy. Why the survival rate of patients with unmutated IgHV is worse than that of patients with mutated IgHV is unknown. CLL cells with unmutated IgHV were thought to originate from naïve B lymphocytes, whereas CLL cells with mutated IgHV were thought to arise from B cells that have undergone somatic hypermutation (SHM). Cell surface protein expression profile and gene expression studies showing that all CLL cells, regardless of their IgHV mutation status, are of postgerminal center origin, negated this hypothesis. We hereby propose that all CLL cells undergo SHM and their proliferation rate determines their IgHV mutation status. DNA breaks, accumulated during SHM, are restored by various DNA repair mechanisms. In rapidly dividing cells DNA breaks are repaired by the efficient high-fidelity homology-directed DNA repair apparatus, whereas in slowly dividing cells they are repaired by the inefficient low-fidelity nonhomology end-joining repair mechanism. Accordingly, a low IgHV mutation rate is found in rapidly dividing cells whereas a high mutation rate is typically found in slowly dividing cells. Thus, the proliferation rate of CLL cells determines the IgHV mutation status and patients’ clinical outcome.