〔Abstract〕 Objective To investigate the differential expression and distribution of forkhead-box O class 1(FoxO1) protein in lung cancer and esophageal carcinoma tissues, and to explore the association between subcellular distribution and function of FoxO1. Methods Immunohistochemistry was conducted to evaluate the cytoplasmic expression of FoxO1 in tissue microarray(TMA) of clinical lung cancer and esophageal carcinoma samples. The differential expression of FoxO1 in pattern and distribution between cancer and normal tissues were analyzed. Statistical analyses were performed to explore the correlation between the cytoplasmic FoxO1 protein and the clinicopathological characteristics of patients; the expression and localization of FoxO1 in lung cancer cell line A549 and esophageal carcinoma cell line Eca109 were analyzed by immunofluorescence; the siRNA knockdown and subsequent colony formation experiments were performed to investigate the effect of FoxO1 on tumor cell proliferation. Results The cytoplasmic FoxO1 expression in lung cancer and esophageal carcinoma tissues was higher than that in the corresponding paracancerous tissues(P<0.05), and there was no significance in the differential expression of cytoplasmic FoxO1 between lung squamous cell carcinoma and lung adenocarcinoma tissues; FoxO1 was dominantly localized to the cytoplasm, with less in the nucleus(P<0.05); the FoxO1 knockdown in A549 and Eca109 cells inhibited cell proliferation. Conclusion The expression of FoxO1 in lung cancer and esophageal carcinoma tissues is upregulated in the cytoplasm, while downregulated in the nucleus; the increased cytoplasmic expression of FoxO1 in cancer cells may promote cell proliferation.