The tumor suppressor gene Ras association domain family 1A (RASSF1A) is highly methylated in a wide range of human sporadic tumors. The current study investigated the hypermethylation of RASSF1A, the expression of RASSF1A protein, and the correlation between these and the clinicopathological features of gallbladder (GB) cancer in Korean patients. Formalin-fixed, paraffin-embedded tumors and non-neoplastic GB tissues (22 carcinomas, 8 adenomas, 26 normal epithelia) were collected from patients who had undergone surgical resection. The methylation status of two regions of the RASSF1A CpG island was determined by methylation-specific PCR (MSP), and the expression of RASSF1A protein was examined by immunohistochemistry using tissue microarrays. The K-RAS mutation was analyzed by direct sequencing. Methylation of the RASSF1A promoter (region 1) was detected in 22.7% (5/22) of carcinomas, 12.5% (1/8) of adenomas, and 0% (0/26) of normal gallbladder epithelia (P = 0.025). Methylation of the first exon (region 2) was found in 36.4% (8/22) of carcinomas, 25.0% (2/8) of adenomas, and 8.0% (2/26) of normal gallbladder epithelia (P = 0.038). K-RAS mutations were present in 4.5% (1/22) of carcinomas and 25% (2/8) of adenomas. RASSF1A methylaton was not associated with clinicopathological factors or K-ras mutation. Reduction or loss of RASSF1A expression was observed in most methylated adenocarcinomas. Three RASSF1A-expressing human biliary tract cancer cell lines examined contained unmethylated promoters and exons 1. These results suggest that downregulation of RASSF1A expression by DNA hypermethylation may be involved in GB carcinogenesis.