The STK11 is a tumor suppressor gene located on chromosome 19p13.3. The encoded protein has serine--threonine kinase activity. Functionally, STK11 regulates cellular energy metabolism and cell polarity by activating AMP-activated protein kinase (AMPK) and other members of the AMPK family. Germline mutations in the STK11 gene are responsible for Peutz--Jeghers syndrome, an autosomal dominant disorder with variable clinical phenotype and increased risk of some cancers. Somatic mutations of STK11 gene are reported in several tumors including lung cancers. Studies have demonstrated STK11 inactivation is a common event and may be involved in the development of sporadic lung adenocarcinoma. Inactivation mutations of STK11 are found in 15% of primary lung adenocarcinomas and 33% of large cell neuroendocrine carcinomas of the lung. Clinical relevance of these alterations and impact on disease progression and patient survival needs to be fully elucidated.