We report a case of prolonged severe hypogammaglobulinemia after rituximab combined chemotherapy for follicular lymphoma. Although the patient's globulin level was within the normal limits before treatment, the level of IgG dropped below 100 mg/dL, and both IgA and IgM became undetectable after treatment, and the levels have shown no changes for 6 years despite recovery of peripheral B cell counts. Phenotypic analysis of B cells revealed a reduction of class-switched CD27+IgM-IgD- memory B cells below 0.5% and overexpression of CD95. On the other hand, we observed the predominance of memory T cell subsets in both of CD4+ and CD8+ T cells as the result of reduction of naïve T cells. These increased memory T cells overexpressed activation markers such as CD69, CD95, and HLA-DR. Furthermore, the patient's B cells failed to differentiate into memory B or plasma cells in the presence of IL-6, IL-10, IL-15, and BAFF in vitro in comparison with those from healthy controls and showed significant impairment of IgG production. These findings suggest that rituximab combined chemotherapy may induce persistent differentiation arrest and apoptosis of B cell lineage with alteration of T lymphocyte homeostasis resulting in pan-hypogammaglobulinemia.