The DNA methylation alterations occurring in human cancers have two types: global DNA hypomethylation and site-specific CpG island promoter hypermethylation. Recently, to assess global DNA methylation, long interspersed nucleotide element 1 (LINE-1) retrotransposons, constituting a substantial portion of the human genome, attracts much attention. The aim of the current study was to clarify the significance of LINE-1 methylation level for epigenetic field defects and the relationships among LINE-1 methylation level in gastric mucosae, clinical and pathological features, including infection by Helicobacter pylori (H. pylori), a bacterium implicated in gastric cancer. By bisulfite-PCR pyrosequencing, we quantified the LINE-1 methylation levels in noncancerous gastric mucosae and cancer tissues from 87 gastric cancer patients, in gastric mucosae from 17 autopsied individuals without gastric cancers and in 20 gastric fresh frozen samples from non-gastric cancer patients. LINE-1 methylation in the noncancerous gastric mucosae of gastric cancer patients was significantly higher than in cancer tissues (P = 0.0006), but significantly lower than in the gastric mucosae of the autopsied individuals (P = 0.026), suggesting the formation of epigenetic field defect in noncancerous gastric mucosae. Moreover, LINE-1 hypomethylation of noncancerous gastric mucosae in gastric cancer patients significantly correlated with H. pylori infection (P = 0.037). We prospectively confirmed the similar result in 20 non-gastric cancer patients (P = 0.010). LINE-1 hypomethylation of gastric mucosae significantly correlated with H. pylori infection, supporting the potential of LINE-1 methylation level as a surrogate marker of epigenetic field defects for gastric cancer cancerization, particularly induced by H. pylori.