CYP1A2 activity has been demonstrated to be bimodally or trimodally distributed in several populations, consistent with a codominant or recessive functional genetic polymorphism. However, studies aimed at identifying polymorphisms inCYP1A2have not yet adequately accounted for this distribution pattern. To search for functional polymorphisms, we performed genome-walking, polymerase chain reaction (PCR) sequencing, and cloning, and identified three novel polymorphisms in the 5′ flanking region ofCYP1A2: a T−3591G substitution, a G−3595T substitution, and a T−3605insertion. The frequency of the T−3591G substitution was determined by a PCR-restriction fragment length polymorphism assay, and found to be significantly higher (P< 0.0001) in Taiwanese (allele frequency 0.128,n= 125) compared to Caucasians (0.017,n= 87) or African Americans (0.024,n= 104). The functional consequence of the T−3591G and the G−3595T substitutions was determined by site-directed mutagenesis followed by transient transfection experiments. The T−3591G mutation was shown to be nonfunctional, while although the G−3595T mutation appeared to result in an increase in promoter activity, this was only to a small degree and therefore unlikely to be importantin vivo. In addition, we report 532 bases of 5′ flanking sequence further upstream than that reported to date, and four sequence discrepancies compared to the original published sequence (G−3649C, ΔT−3650, ΔA−4072, and C−4093ins).