Fifteen cases of primary pulmonary malignant lymphoma (M.L.) were studied using strict criteria to distinguish them from secondary pulmonary manifestations. All were M.L.'s of the lymphoid B type. Diagnosis on the basis of non specific clinical, X-ray and endoscopic findings if often difficult but can be facilitated by the systematic use of immunocytochemical and ultrastructural techniques. Their evolution is in good agreement with the prognostic data given by the Kiel-Lennert histopathological classification. 14 of the patients had M.L. of low grade malignancy; the one case of high grade malignancy was quickly fatal. A few cases appeared as a complication of a preexisting dysimmune disease (Pigeon breeder's disease, Gougerot-Sjogren disease, Liebow's lymphomatoid granulomatosis).