Background: Adult onset Still's disease (AOSD) is a rare systemic inflammatory disorder of unknown aetiology, frequently accompanying multiple lymphadenopathy. It often mimics malignant lymphoma, and immunohistochemical and molecular studies are needed for definite diagnosis. Aims: To aid in diagnosis and understand the pathogenesis of the disease by clarifying lymph node (LN) pathology in AOSD. Methods: Thirteen biopsies (one follow up biopsy) and medical records of 12 patients were reviewed. Immunohistochemistry, polymerase chain reaction for T cell receptor c chain (TCRc) and immunoglobulin heavy chain gene rearrangement, and Epstein-Barr virus in situ hybridisation were performed. Results: Histologically, LN lesions were classified into four patterns. The most common (six biopsies) showed paracortical hyperplasia, with prominent vascular proliferation, scattered large B/T immunoblasts, and infiltration by reactive lymphocytes and inflammatory cells. In the second pattern (two biopsies), paracortical hyperplasia was accompanied by massive sinus histiocytosis and S-100 positive histiocyte aggregates. The third pattern (three patients) showed an exuberant immunoblastic reaction, in the form of patchy/diffuse infiltration of large T immunoblasts with high mitotic activity, although clonal rearrangement of the TCRc gene was not detected. The fourth pattern showed distinct follicular hyperplasia (two cases). One patient with a follow up biopsy showed a pattern change from pronounced follicular hyperplasia to atypical paracortical hyperplasia. Conclusions: AOSD LN lesions show a dynamic histological spectrum, including atypical paracortical hyperplasia, burnt out histiocytic reaction, exuberant immunoblastic reaction, and follicular hyperplasia. During the course of disease, LN reactivity changes and mixed B and T cells are involved in the pathogenesis.