This paper described the first clinical study of lung tumor scanning by positron emission tomography (PET) using C-11-labeled L-methionine (11C-L-Met). Tumor images were clearly visualized by high contrast in eight lung cancer patients and also in a patient with a silicotic nodule. Quantitative evaluations of methionine uptake in tumor tissue and normal tissue by comparing differential uptake ratios suggested that the extent to which 11C-L-Met accumulates in a tumor is closely correlated to the tumor's viability such as benign or malignant, viable or necrotic. 11C-L-Met is considered to be an effective tumor marker for PET diagnosis which represents increased amino acid metabolism of tumors in the mediastinum and lung.