We present a case of primary non Hodgkin's lymphoma of the lung parenchyma. The patient was admitted to our department with diffuse bilateral consolidations on chest x-ray. He had a history of one episode of coughing up blood stained sputum associated with chest tightness and high fever two weeks before admission. He underwent an open lung biopsy after the failure of other non invasive investigation to put a finαλ diagnosis. The pathology report referred to lesions compatible with an interstitial lymphohyperplastic process that usual immunochemical assays failed to identify its nature. Finally the application of PCR technique put the diagnosis of a low grade B cell BALT lymphoma by the demonstration of cell monoclonicity on the histologic specimen. It is worth mentioning that the severe immunodeficiency of this patient is not commonly reported in BALT lymphoma cases. Pneumon 2000, 13 (3): 231-235