CYP1A1 expression in various tissues of 43 gastric cancer cases was analyzed with frozen tissue array-based immunohistochemical staining (IHC), RT-PCR, Western blot analysis and EROD (7-ethoxyresourfin-O-deethylase) assay. CYP1A1 was detected immunohistochemically in 86% (37/43) of gastric cancers, in both 57% (4/7) of atrophic gastritis and intestinal metaplasia and in 7.7% (1/13) of non-cancerous mucosa. RT-PCR and Western blot analysis revealed coincident results with that of IHC and ruled out possible concurrent CYP1A2 expression. EROD assay showed increased CYP1A1/1B1 activity in either cancer or premalignant tissues but not in non-cancerous ones. The frequencies of CYP1A1 expression are significantly different between non-cancerous and premalignant (P<0.025) or cancer groups (P<0.005), suggesting that CYP1A1 is expressed at relatively early stage of gastrocarcinogenesis and exerts its effects throughout the stepwise oncogenic processes.