Recent studies have shown that cancer cells surviving in a microenvironment characterized by hypoxia, a low pH, and a low glucose level have the ability to adapt to these adverse conditions. We measured the pH in the central tumor area of primary lung cancer, and evaluated its association with clinicopathological factors. There was a negative correlation between the tumor size and pH; with an increase in the tumor size, the pH decreased. In addition, pH in lung adenocarcinoma was negatively correlated with the expression of tumor markers, although the correlation was not significant. Cancer cells grow at a markedly low pH compared with the physiological environment. There is a possibility that this low pH is a microenvironment that is appropriate rather than adverse for the proliferative ability of cancer cells.