We previously have shown the aberrant overexpression of stratifin (SFN, 14-3-3 ς) in lung adenocarcinoma. Although SFN is known to facilitate tumor cell proliferation, the mechanism that underlies its aberrant expression has remained unclear. SFN, the downstream target of p53, often has been reported to be hypermethylated and subsequently silenced in certain cancers; however, its hypomethylation-linked reactivation has not yet been validated. In this study, we investigated the DNA methylation status of the SFN promoter region using 8 lung cancer cell lines and 32 specimens of adenocarcinoma tissue. Real-time methylation-specific PCR analysis showed that although both normal lung tissue and adenocarcinoma in situ bore a completely methylated SFN promoter, the promoter region in almost all invasive adenocarcinomas was at least partially methylated. The expression of SFN and its level of methylation were correlated strongly. Furthermore, statistical analysis revealed that the level of methylation became reduced with progression of the pathologic stage, although no clear relationship between methylation level and p53 abnormality was found. These results suggest that methylation-related silencing of SFN occurs in both normal lung tissues and adenocarcinoma in situ, and that demethylation of the SFN promoter participates in the aberrant expression of SFN in invasive adenocarcinoma cells, independently of p53 alteration. This novel finding might be informative for clarifying the mechanism that underlies the progression of early lung adenocarcinoma.