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Despite a recent decline, tuberculosis (TB) infection is still a frequent diagnosis in Portugal. Adenosine deaminase (ADA) measurement has become an important tool in the timely diagnosis of this infection. However, ADA elevation in bodily fluids is not pathognomonic of TB infection. We present the case of a 70-year-old woman, undergoing treatment for pleural TB, diagnosed based on elevated ADA levels in a pleural effusion. Due to worsening symptoms she was readmitted, and the previous diagnosis was reconsidered. Thoracocentesis was repeated and cytometry analysis of the fluid was performed, showing the presence of diffuse large B cell lymphoma (DLBCL). DLBCL is the most frequently occurring non-Hodgkin lymphoma (NHL). Pleural involvement is rare in the initial stages. ADA elevation >250 U/l should raise suspicion of malignancy, especially in association with markedly elevated LDH levels. The purpose of this case report is to highlight that in the absence of microbiologic or histologic confirmation, a presumptive TB diagnosis should not be lightly made, and alternative diagnoses should be systematically ruled out., Learning Points: Elevated ADA levels are not pathognomonic of TB infection.Lymphocytic pleural effusion requires a systematic diagnostic approach.Very high levels of ADA should arouse suspicion of lymphoma., Competing Interests: Conflicts of Interests: The Authors declare that there are no competing interests., (© EFIM 2020.)