The first possibility considered in the etiology of large lung masses is neoplastic lesions. The differential diagnoses of these masses include bronchogenic carcinoma, pulmonary sarcoma, primitive neuroectodermal tumor etc. Primary or secondary pulmonary parenchymal lymphomas presenting as large mass is distinctly rare. We share the case of a young lady who presented with a large left lung mass almost entirely replacing the left lung parenchyma, with associated intrathoracic lymphadenopathy. On evaluation she was proved to have primary mediastinal large B-cell lymphoma. Treatment with an aggressive chemotherapy regimen led to complete remission of the parenchymal and nodal disease. The uncommon radiological presentation and the excellent therapeutic response despite huge tumor load merit clinical attention.