Primary carcinoma of the lung is not a rare disease but constitutes about 6 to 8 per cent of all carcinomas.1In frequency it ranks next to malignant conditions of the gastro-intestinal tract. Twenty years ago only 5 per cent of the cases were diagnosed clinically; today about 50 per cent are recognized during life. After a ten year study of this subject we are convinced that at least 90 per cent of the cases can be diagnosed. The clinician who is familiar with the pathologic and clinical manifestations of primary lung carcinoma can recognize most cases from the history, physical examination and roentgen study. In some cases a bronchoscopic examination, injection of iodized oil or artificial pneumothorax will be necessary. The diagnosis can be confirmed by a biopsy of the frequently enlarged cervical or axillary lymph nodes, by microscopic examination of pleural exudates or pieces of tissue in