The differential diagnosis of pulmonary tuberculosis and lymphoma with pulmonary infiltration is very difficult, given their similar clinical characteristics. We report a 59 year old female with weight loss, fever, dyspnea and cough of several months of duration. She had a cavitated mass in lung imaging. A positive conventional PCR lead to the diagnosis of tuberculosis, but she had negative smears and cultures for Mycobacterium tuberculosis. The patient did not respond to treatment and her clinical condition worsened. A peripheral lymph node biopsy confirmed the presence of a diffuse large B cell non-Hodgkin lymphoma. Bone marrow pathology showed non caseating granulomas, again with negative microbiological tests for M. tuberculosis. Findings in the bone marrow were interpreted as a secondary sarcoid reaction to cancer and PCR results as a false positive. The lymphoma was treated, achieving complete remission. This case highlights the importance of the differential diagnosis between these two entities.The differential diagnosis of pulmonary tuberculosis and lymphoma with pulmonary infiltration is very difficult, given their similar clinical characteristics. We report a 59 year old female with weight loss, fever, dyspnea and cough of several months of duration. She had a cavitated mass in lung imaging. A positive conventional PCR lead to the diagnosis of tuberculosis, but she had negative smears and cultures for Mycobacterium tuberculosis. The patient did not respond to treatment and her clinical condition worsened. A peripheral lymph node biopsy confirmed the presence of a diffuse large B cell non-Hodgkin lymphoma. Bone marrow pathology showed non caseating granulomas, again with negative microbiological tests for M. tuberculosis. Findings in the bone marrow were interpreted as a secondary sarcoid reaction to cancer and PCR results as a false positive. The lymphoma was treated, achieving complete remission. This case highlights the importance of the differential diagnosis between these two entities.