 At the Brigham and Womens Hospital Anticoagulation Service, we identified 73 patients for CYP2C9 genotyping, which we assessed with PCR amplification and restriction enzyme digestion analysis of DNA isolated from circulating leukocytes. The CYP2C9 polymorphisms independently predicted low warfarin requirements after adjusting for body mass index, age, acetaminophen use, and race. At least one polymorphism was present in every patient requiring 1.5 mg or less of daily warfarin. We did find higher rates of excessive anticoagulation but did not observe higher bleeding rates in patients with these polymorphisms. 