We previously reported that loss of heterozygosity (LOH) in tuberous sclerosis 1 (TSC1)-gene-associated region on chromosome 9q34, was frequently observed in adenocarcinoma (AC) of the lung and its putative precursor lesion, atypical adenomatous hyperplasia (AAH). Some novel tumor suppressor gene for AC of the lung may be present in this region, and the TSC1 gene located on chromosome 9q34 is one candidate. The aim of this study was to determine whether the TSC1 gene itself acts as a tumor suppressor gene in the pathogenesis of AC of the lung. DNA extracted from frozen tumor tissue was used to screen 47 ACs of the lung for the presence of mutations in 21 coding exons of the TSC1 gene. When using these bulk tissue specimens, three different types of mutations were detected in five ACs. These positive samples were then submitted to laser capture microdissection to selectively collect tumor cells, DNA extraction and finally analysis of LOH and mutations of the TSC1 gene. Mutations were detected in three ACs, and LOH was detected in another AC. No mutations or LOH was detected in the other AC. Loss of heterozygosity and mutations were not detected simultaneously in the same tumor. We concluded that the TSC1 gene itself does not act as a tumor suppressor gene in the pathogenesis of AC of the lung in accordance with classical Knudsen's two-hit hypothesis. However, the frequent LOH in AC and AAH shown in our previous studies indicate the following possibility in the etiology of AC of the lung: the presence of a novel tumor suppressor gene close to the TSC1 gene, aberrant promoter methylation, or haploid insufficiency of the TSC1 gene.