A case of small and large cells non Hodgkin lymphoma with an unusual involvement of the lung is presented. The tumor was located in the inferior lower lobe of the right lung and coincided with hemorrhagic pleural effusion. The cytopathological study of the fluid was negative for neoplastic cells, so were the aspiration biopsy of the lung and the pleural biopsy. The transmission and scanning electron microscopic study of the pleural fluid demonstrated lymphoid atypical cells. The patient was treated by resection of the lower right lobe and pleurectomy. The difficulties in the differential diagnosis of this type on non Hodgkin lymphoma are discussed. The importance of scanning and transmission electron microscopy is stressed when the light cytologic studies are negative in processes suggestive of being neoplasms. After one year of the lobectomy and treatment with vincristine-epirubicin-cyclophosphamide and methyl-prednisolone, the patient is in good condition without evidence of neoplastic activity.