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Background: Musculoskeletal involvement occurs in 25% of patients with non-Hodgkin's lymphoma (NHL). Primary lymphoma in the joint is rare. It can present as a bone lesion or as atypical soft tissue proliferation. NHL has an increased incidence in patients with autoimmune rheumatic diseases., Case Presentation: We present a case in which non-Hodgkin's lymphoma was found coincidentally in the synovium during knee joint replacement surgery in a 69-year old woman with rheumatoid arthritis. Pigmented, vitreous tissue was resected, which turned out to be a diffuse large B-cell lymphoma after histological examination. The coincidental intraoperative finding of intra-articular non-Hodgkin's lymphoma was reported twice before, presenting as synovial proliferation in elbow and shoulder surgery. In a few other cases non-Hodgkin's lymphoma presented most often in the knee, as a bone lesion or, when soft tissue was involved, as arthritis., Conclusion: Non-Hodgkin's lymphoma should be considered in patients with autoimmune rheumatic diseases. In case of persistent arthritis, non-respondent to anti-inflammatory drugs, a biopsy might be warranted. Moreover, when arthroscopy or arthrotomy is planned, any atypical tissue should be sent for histological analysis. Early diagnosis of NHL can contribute to improved outcome of its rapidly developing treatment options.