Warfarin is a commonly used prophylactic agent for the prevention of thromboembolic disease. We hypothesized that racial background influenced warfarin dosage, and tested this by recording the international normalized ratio (INR) in 867 patients aged 40-90 routinely passing through our Anticoagulation Service whose target INR was 2-3. Mean (95% confidence interval) dose was 4.1 (4.0-4.2) mg/d in 737 Caucasians, 5.5 (4.9-6.1) mg/d in 72 Asians, and 6.7 (5. 8-7.6) mg/d in 58 Afro-Caribbeans (P < 0.05) between each group). In a subgroup of 302 (41 Asians, 22 Afro-Caribbeans, 239 Caucasians), body mass index did not influence warfarin use. Despite small numbers, we conclude that racial background, but not body mass index, is a determinant of warfarin dosage. The reasons for this could be genetic, cultural (diet related), or both.