Abstract Background: Lung cancer is the leading cause of human cancer death worldwide. DNA methylation affects the aggressiveness of human malignancies. Cancers with CpG island methylator phenotype (CIMP), a distinct group with extensive DNA methylation, show characteristic features in several types of tumors. However, the existence and characteristics of CIMP in lung adenocarcinomas (AdCas) from the global point of view has remained elusive. The aim of this study is to clarify the significance of DNA methylation, especially the clinical impact of CIMP, in AdCas. Methods: Using genome-wide microarray analysis, DNA methylation profiling of 41 AdCas were initially examined. DNA methylation status of six CIMP markers newly identified by microarray analysis was further estimated in a validation set of 87 AdCas by bisulfate pyrosequencing. A total of 128 AdCas were examined for the significance of CIMP. In addition, responsiveness to DNA methylation inhibitor was examined in AdCa cell lines with different CIMP status. Results: Consensus clustering analysis based on DNA methylation showed that AdCas were classified into three subgroups, clusters 1, 2, and 3 (average of 767, 486, and 319 methylated genes in each subgroup, respectively; P<0.001), indicating existence of CIMP in AdCas. DNA methylation status of six CIMP markers was examined in a total of 128 AdCas and revealed that 10 (7.8%), 40 (31.3%), and 78 (60.9%) were classified as CIMP-high (CIMP-H), CIMP-low, and CIMP-negative, respectively. Notably, CIMP-H AdCas were strongly associated with wild type EGFR, males, and heavy smokers (P=0.0089, 0.0047, 0.0036, respectively). In addition, CIMP-H was significantly associated with worse prognosis; especially among male smokers, CIMP-H was an independent prognostic factor (HR 1.7617, 95%CI 1.0030-2.9550, P=0.0489). The existence of CIMP in AdCas was supported by the deposited data, the Cancer Genome Atlas dataset and mRNA expression profile dataset as well. Intriguingly, analysis of AdCa cell lines revealed that CIMP-positive AdCa cell lines were more sensitive to a DNA methylation inhibitor than CIMP-negative ones regardless of EGFR mutation status. Conclusions: Our data demonstrate that CIMP in AdCas appears to be a unique subgroup that has distinct clinical traits from other AdCas. CIMP classification using our six-marker panel has implications for personalized medical strategies for lung cancer patients; especially, DNA methylation inhibitor might be of therapeutic benefit to patients with CIMP-positive tumors. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5003. doi:1538-7445.AM2012-5003