Immunoglobulin (Ig) G4-related disease is a recently confirmed clinical entity with several unique clinicopathological features. Here we report a case of rheumatoid lymphadenopathy mimicking IgG4-related disease. The patient was a 63-year-old woman who had been treated for rheumatoid arthritis (RA) for six years. The patient noted cervical lymphadenopathy. Upon radiological examination, systemic lymphadenopathy was detected, and enlarged right brachial lymph node biopsy was performed. Histologically, the lymph node showed marked follicular hyperplasia and interfollicular plasmacytosis without eosinophil infiltration. Although the histological findings were compatible with rheumatoid lymphadenopathy, numerous plasma cells were IgG4⁺ (IgG4⁺/IgG⁺ plasma cell ratio > 50%). However, laboratory findings revealed elevation of C-reactive protein level, polyclonal hyper-γ-globulinemia, anemia, and hypoalbuminemia. These findings were compatible with hyper-interleukin (IL)-6 syndrome, namely, RA. It is known that hyper-IL-6 syndromes, such as multicentric Castleman's disease, RA, and other autoimmune diseases, fulfill the histological diagnostic criteria for IgG4-related disease. Therefore, hyper-IL-6 syndromes and IgG4-related disease cannot be differentially diagnosed by immunohistochemical staining alone. In conclusion, rheumatoid lymphadenopathy sometimes occurs with abundant IgG4⁺ plasma cells, which is required for the differential diagnosis of IgG4-related disease. [J Clin Exp Hematopathol 52(1) : 57-61, 2012]