The authors reported 33 cases of malignant lymphoma in the lung observed over a period of 6 years on the Pathological Department of the Beaujon Hospital. Whatever their cytological, lymphocytic, lymphoblastic or reticular type (Hodgkin's disease was excluded from this study), they may be grouped in two main pathological categories: 1) Malignant lymphoma involving the lung and mediastinum without extra-thoracic lesions detected at the time of diagnosis (4 cases). In 3 cases, the disease had been present for 2 to 6 months. In 1 case the course was favourable for 5 years then the patient developed acute lymphoblastic leukemia. 2) The pulmonary lesions observed during general infections, whether as presenting symptoms in 9 cases, or simply discovered on autopsy as in 20 cases. Histological examination, essential for the diagnosis of malignant lymphoma, provides no evidence in favour of the primary pulmonary origin of the disease; this diagnosis can only be made after full blood investigations. The prognosis is also difficult to determine for survival for many years may be followed by sudden general malignant blood disease.