A 33-year-old Asian woman presented with fever and neck lymphadenopathy. Laboratory examination revealed microcytic anemia, lactate dehydrogenase elevation, reverse albumin/globin ratio, and red blood cell Rouleaux formation. Serum immunofixation electrophoresis indicated polyclonal hypergammaglobinemia (immunoglobulin G [IgG] 7710 mg/dL) and a normal free light chain ratio. Other autoimmune markers were within normal limits except elevated interleukin-6. Serum IgG4/IgG ratio was low (0.12; IgG4-related disease, >0.4). Plain films of the skeleton revealed no osteolytic lesions. She had been stable and required no active treatments for 3 years. Bone marrow smear revealed atypical plasma cells (30% in total nucleated cells) (panel A) with myeloma-like features such as multiple nuclei (panel B), Russell bodies (panels C and E [arrowhead]) and flame cells (panel D) (panels A-D; original magnification ×1000; Liu stain). Bone marrow biopsy confirmed infiltrates of CD138 + plasma cells (panels E [arrow] and F). κ and λ (panel G) stains indicated no light chain restrictions. Lymph node biopsy (panel H) showed reactive interfollicular plasmacytosis. Human herpes virus-8 staining was negative (panels E and H; original magnification ×1000 [E], ×200 [H], and ×1000 [H, inset], hematoxylin and eosin stain; panels F-G; original magnification ×200, immunohistochemistry).Idiopathic plasmacytic lymphadenopathy with polyclonal hypergammaglobinemia is a rare benign inflammatory disease and should be excluded from other autoimmune diseases, such as multicentric Castleman disease or IgG4-related disease. Clinicians should be aware of benign plasma cell proliferation mimicking malignancy.