The interleukin-mediated Janus kinase (JAK)/STAT pathway plays a crucial role in carcinogenesis. Recently, increased STAT3 activity was found in hepatocellular carcinoma and multiple myeloma in which there was silencing of SOCS-1 (suppressor of cytokine signalling-1) by gene promoter hypermethylation. We investigated the expression level of interleukin-6 (IL-6) and SOCS-1 in gastric cancer cell lines. Expression of SOCS-1 correlated with IL-6 level in most of the cell lines, except for AGS cells in which SOCS-1 was absent despite a high level of IL-6 production. Methylation analysis by methylation-specific polymerase chain reaction and bisulphite sequencing revealed that CpG island of SOCS-1 was densely methylated in AGS cells. Demethylation treatment by 5'aza-deoxycytidine restored SOCS-1 expression and also suppressed constitutive STAT3 phosphorylation in AGS cells. Moreover, methylation of SOCS-1 was detected in 27.5% (11 of 40) of primary gastric tumours samples, 10% (one of 10) of adjacent noncancer tissues but not in any (zero of nine) normal gastric mucosa. Methylation of SOCS-1 also correlated with the loss of mRNA expression in some primary gastric cancers. In conclusion, this is the first report to demonstrate that hypermethylation of SOCS-1 led to gene silencing in gastric cancer cell line and primary tumour samples. Downregulation of SOCS-1 cooperates with IL-6 in the activation of JAK/STAT pathway in gastric cancer.