Abstract. Cytochrome P450 (CYP) 1A1 (CYP1A1) and CYP1B1, dioxin-inducible CYP1s, are associated with carcinogenesis in extrahepatic tissues. CYP1B1 is featured in carcinogenesis of hormone-responsive tissues, where the CYP1B1 level is considerably high. Although aberrant expression of these enzymes is also observed in cancers that are not related to hormone response, their roles in carcinogenesis are not yet fully understood. We examined DNA methylation status of the CpG islands within the 5'-flanking region of the CYP1B1 and CYP1A1 genes in 7 colorectal cancer cell lines and 40 primary colorectal cancers. By bisulfite-modified direct sequencing, CYP1B1 gene methylation was detected in 2 cell lines (SW48 and Caco-2) and 2 (5%) cancers, but not in corresponding normal tissues. Treatment of the cells with 5-aza-2'-deoxycytidine revealed a clear increase in the CYP1B1 mRNA levels in SW48 and Caco-2 cells, while the amount of methylated alleles decreased. Only HT29 cells showed a clear increase in CYP1A1 mRNA, although there were no apparent differences in methylation status among these cell lines. None of these cell lines showed significant change in mRNA levels of aryl hydrocarbon receptor (AhR) and AhR nuclear translocator (ARNT), which are known to directly activate CYP1 transcription. This observation suggested that expression of CYP1B1, but not CYP1A1, was downregulated by promoter methylation rather than decreased expression of AhR/ARNT. In conclusion, CpG methylation of the CYP1B1 promoter region epigenetically regulates CYP1B1 expression during development of some colorectal cancers. Moreover, cancers with aberrant CYP1B1 expression might show altered response to procarcinogen metabolism and chemotherapy.