The inhibitory Fc-γ receptor FcγRIIB, expressed on myeloid and B cells, has a critical role in the balance of tolerance and autoimmunity, and is required for the antiinflammatory activity of intravenous Ig (IVIG) in various murine disease models. However, the function of FcγRIIB and its regulation by IVIG in human autoimmune diseases are less well understood. Chronic inflammatory demyelinating polyneuropathy (CIDP) is the most common treatable acquired chronic polyneuropathy, and IVIG is widely used as a first-line initial and maintenance treatment. We found that untreated patients with CIDP, compared with demographically matched healthy controls, showed consistently lower FcγRIIB expression levels on naive B cells, and failed to up-regulate or to maintain up-regulation of FcγRIIB as B cells progressed from the naive to the memory compartment. Concomitantly, the rare −386C/−120A FcγRIIB promoter polymorphism resulting in reduced promoter activity previously associated with autoimmune phenotypes was overrepresented in CIDP. Also, FcγRIIB protein expression was up-regulated on monocytes and B cells after clinically effective IVIG therapy. Thus, our results suggest that the inhibitory FcγRIIB is impaired at a critical B cell differentiation checkpoint in CIDP, and that modulating FcγRIIB expression might be a promising approach to efficiently limit antibody-mediated immunopathology in CIDP.