After injection of 131I‐MAA (macro‐aggregated human serum albumin), the authors obtained pulmonary scans from 500 patients with various pulmonary diseases. The results of scanning and of standard roentgenographic examination are compared on the basis of 138 cases with definitely diagnosed malignant pulmonary tumors. In all cases with central tumors;, i.e., tumors that were located near the hilus, the scan was abnormal and thus indicated a disorder of pulmonary perfusion. In contrast to this, radiologic examination (thoracic films in two planes and, if indicated, fluoroscopy) only disclosed direct or indirect signs of cancer in 88% of these cases. Therefore, lung scanning as a screening method for central pulmonary tumors is a more sensitive diagnostic tool than standard roentgenographic examination. In cases of peripheral pulmonary tumors, a perfusion disorder which is larger than the radiologically recognizable lesion indicates central metastases.