Leucine aminopeptidase (LAP) activity and its isozyme fraction were assayed in the serum of 34 cases of lung cancer, and were compared with those in the 34 cases of stomach cancer, 12 of breast cancer and 10 of various kinds of nonneoplastic lung disease as well as those of 25 healthy subjects. The cases with those having any abnormality in the result of hepatorenal function tests were excluded from this study. The activity of LAP was remarkably high in the lung cancer and 90% of the patients who showed abnormally high level of the activity was proved to be the lung cancer patient. The ratio of Y fraction in the isozyme of lung cancer was also significantly higher than that in any other group and this was demonstrated in 96% of the cases. Although there was no significant difference in the total activity of LAP among the stages of lung cancer, elevation of the ratio of Y fraction in isozyme was significant in accordance with the stage. This was fact even in the range of the tumor size. As to lymphnode metastasis of lung cancer, increase in both the ratio and activity of the Y fraction was observed in association with the progress of extent of the metastasis, and there was a significant difference in the values between those having metastasis within and beyond the mediastinum. It was demonstrated that the total activity of LAP and the ratio and activity of its Y fraction were higher even in the cases with stomach or breast cancer, but it was observed in the advanced stage in contrast with the lung cancer. Furthermore, the elevation of the ratio was consistently lower than that in the lung cancer. On the basis of the results mentioned above, it would be reasonable to signify a clinical value of assaying the Y fraction of LAP isozyme on the diagnosis and the treatment of lung cancer.