Mutation of the LKB1 gene (also known as STK11) is regarded as a cause of Peutz-Jeghers syndrome. In Caucasian patients, LKB1 somatic mutations occur in approximately one-third of lung adenocarcinomas. The aim of the present study was to examine the LKB1 gene in Japanese patients with lung cancer and to evaluate its clinical and pathological implications. We sequenced the LKB1 gene in 22 lung cancer cell lines and 100 Japanese patients with lung cancer (including 81 adenocarcinomas, 14 squamous cell carcinomas and five other histological types) who had undergone curative pulmonary resection. We also determined expression levels of the LKB1 gene by quantitative real-time reverse transcription-polymerase chain reaction and correlated these results with the clinical and pathological features of patients. Among the 22 cell lines, four had mutations and three of these were in adenocarcinoma cells. Of 100 primary lung cancers, only three had LKB1 gene mutations (3%). All of them were male smokers with adenocarcinomas. Hence, when confined to this subset of patients, the mutation frequency was 9% (3/33). No significant correlation was observed between the expression level of LKB1 and patient clinicopathological features. In conclusion, LKB1 gene mutations were relatively rare in Japanese patients with lung cancer compared with Caucasian patients. LKB1 gene mutations appear to be frequent in male, smoking patients of Caucasian origin, in contrast to EGFR or HER2 mutations that are frequent in non-smoking, female patients of Asian origin.