Abstract: It was measured that serum glucose phosphate isomerase (GPI) activity and carcinoembryonic antigen (CEA) of 300 subjects, which were divided into 4 groups at the same time. Serum GPI and CEA were significantly higher in patients with lung cancer and malignant diseases than those in the control group of healthy subjects and the benign diseases group. A 66.7% positive rate in the lung cancer group and 49.5% in other malignant disease groups were reported, respectively. The positive rates in small cell lung cancer and squamous cell lung cancer were more than 80%, especially. GPI sensitivity was higher than CEA, and CEA specificity was higher than GPI. The diagnostic efficiency and correct diagnostic rate can be increased by combining both. Evaluating GPI as a lung cancer marker was discussed.