Cytochrome P4501B1 (CYP1B1) is involved in the activation of many carcinogens and in the metabolism of steroid hormones, including 17β-oestradiol (E2) and testosterone. We report a significant difference in the allele frequencies of two point mutations in the coding region of theCYP1B1gene among Caucasian (n= 189), African-American (n= 52) and Chinese (Linxian) (n= 109) populations. A (C to G) transversion at position 1666 in exon 3, which results in an amino acid substitution of Leu432to Val, was present in African-Americans with an allele frequency for Val432of 0.75, in Caucasians of 0.43, and in Chinese of 0.17. A (C to T) transition at position 1719 in exon 3, with no amino acid change (Asp449), appeared to be closely linked with the Val432variant. Results using human lung microsomal preparations from individuals with theCYP1B1Val/ValandCYP1B1Leu/Leugenotypes indicate thatVal432variant may be a high activity allele and thus may contribute to the inter-individual differences inCYP1B1activity. Because CYP1B1 is involved in hormone and carcinogen metabolism, and given the disparate rates of prostate cancer among ethnic groups, we also evaluated the association of theCYP1B1Leu432Val polymorphism with prostate cancer risk in a pilot case–control study. Among Caucasians, 34% of men with cancer (n= 50) were homozygous for the Val432polymorphism, while only 12% of matched control subjects (n= 50) had this genotype. These preliminary data indicate that genetic polymorphisms in CYP1B1 might play an important role in human prostate carcinogenesis.