Hypogammaglobulinemia is a common complication of chronic lymphocytic leukemia (CLL), but the significance of immunoglobulin G (IgG) subclass deficiency is unknown. We analyzed the prevalence of immunoglobulins G, A and M, IgG subclass deficiency and infection in 150 patients with CLL. Low IgG, IgA and IgM levels were observed in 27.3%, 30.7% and 56.7% of patients, respectively. IgG subclass deficiency was frequent, with reduced IgG1, IgG2, IgG3 and IgG4 in 28%, 19.3%, 52% and 22.7% of patients, respectively. IgG subclass deficiency (total 64.6%) and hypogammaglobulinemia (27.3%) were more prevalent than clinically significant infection (16%). Recurrent or significant infections were seen in 24 patients (16%), of whom 50% had hypogammaglobulinemia but 100% had at least one IgG subclass deficiency, indicating that half the patients with infection had IgG subclass deficiency but normal total IgG level. Deficiencies of IgG3 and IgG4 were statistically associated with infection risk. Normal immunoglobulin and IgG subclass levels were seen in 26 patients (17%) and none had infections. IgG subclass deficiency is commonly observed in patients with CLL with both normal and reduced total IgG levels, and is associated with infection. Screening patients with CLL for IgG subclass deficiency may be a useful adjunct in stratifying their infection risk.