ID4 gene is a member of the inhibitor of DNA-binding (ID) family, which inhibits DNA binding of basic helix-loop-helix transcription factors. Certain human primary breast cancers reportedly have low or no expression of ID4 protein, but its role in carcinogenesis and cancer progression is unknown. To determine its possible role, we examined epigenetic inactivation of ID4 gene by promoter hypermethylation in human breast cell lines and T1 breast cancer tissues. Methylation status of ID4 promoter CpG island was assessed by methylation-specific PCR (MSP); ID4 mRNA level was assessed by quantitative real-time RT-PCR. Of eight cell lines, two were fully methylated, four were partially methylated, and two were not methylated. ID4 mRNA level was suppressed in fully methylated cell lines. ID4 hypermethylation was observed in 16 of 24 (67%) node-positive and seven of 36 (19%) node-negative T1 primary breast cancers matched by patient age and tumor diameter. It was a significant risk factor for nodal metastasis (OR 13.1, P=0.0004). ID4 mRNA level was suppressed in hypermethylated cancer specimens (P=0.014). ID4 may play an important suppressive role in tumor progression, and its silencing by hypermethylation may increase the risk of regional lymph node metastasis.