Background: The CCAAT/enhancer binding protein delta (CEBP) is a member of a highly conserved family of basic region leucine zipper transcription factors. It has properties consistent with a tumour suppressor; however, other data suggest that CEBP may be involved in the metastatic process.Methods:We analysed the expression of CEBP and the methylation status of the CpG island in human breast cancer cell lines, in 107 archival cases of primary breast cancer and in two series of metastatic breast cancers using qPCR and pyrosequencing.Results: Expression of CEBP is downregulated in primary breast cancer by site-specific methylation in the CEBP CpG island. Expression is also downregulated in 50% of cases during progression from primary carcinoma to metastatic lesions. The CEBP CpG island is methylated in 81% metastatic breast cancer lesions, while methylation in the CEBP CpG island in primary cancers is associated with increased risk of relapse and metastasis.Conclusion:CCAAT/enhancer binding protein delta CpG island methylation is associated with metastasis in breast cancer. Detection of methylated CEBP genomic DNA may have utility as an epigenetic biomarker of primary breast carcinomas at increased risk of relapse and metastasis.