Frequent losses of chromosome 19p have recently been observed in sporadic lung adenocarcinomas, targeting the location of a critical tumor suppressor gene. Here we performed fine mapping of the short arm of chromosome 19 and found that the LKB1/STK11 gene mapped in the minimal-deleted region. Because germ-line mutations at LKB1/STK11 result in the Peutz-Jeghers syndrome and an increased risk of cancer, we performed a detailed genetic screen of the LKB1/STK11 gene in lung tumors. We detected a high frequency of somatic alterations (mainly nonsense mutations) in primary lung adenocarcinomas and in lung cancer cell lines. Thus, our findings demonstrate for the first time that LKB1/STK11 inactivation is a very common event and may be integrally involved in the development of sporadic lung adenocarcinoma.