Lung cancer, the leading cause of cancer-related death, still lacks reliable biomarkers. Apurinic/apyrimidinic endonuclease 1/Ref-1 is a multifunctional protein involved in the base excision repair of DNA damaged by oxidative stress or alkylating compounds, as well as in the regulation of multiple transcription factors. To validate apurinic/apyrimidinic endonuclease 1/Ref-1 as a biomarker for prediction of lung cancer recurrence, we studied 42 patients who received curative resection and mediastinal lymph node dissection for stage I non-small-cell lung cancer. They were divided into 2 groups based on recurrence, and compared by immunohistochemistry staining of paraffin-embedded tissues and Western blot analysis. Immunohistochemical staining showed a significant difference between the cytoplasm and nucleus in patients who had a recurrence compared to those with nonrecurrent adenocarcinoma. In Western blot analysis, the recurrent adenocarcinoma group showed increased expression of apurinic/apyrimidinic endonuclease 1/Ref-1 in cytoplasm, nucleus, and in total. This indicates that apurinic/apyrimidinic endonuclease 1/Ref-1 is unregulated in recurrent stage I adenocarcinoma. For clinical application as a prognostic marker for non-small-cell lung cancer, further investigation into the role of apurinic/apyrimidinic endonuclease 1/Ref-1 in carcinogenesis is needed in an expanded prospective study.