It has been suggested that aging enhances the pharmacologic effect of warfarin, but there is little information about the effects of warfarin in aging minority populations. The authors examine the response of an aging Hispanic population to warfarin. Charts in their anticoagulation clinic were retrospectively examined for the following information: age, sex, weight, duration of anticoagulant therapy, number of medical problems, number of medications, number of minor or major bleeding episodes, prothrombin time, warfarin dose, and international normalized ratio (INR). The dose-adjusted prothrombin time ratio (PTR) and dose-adjusted INR were calculated by dividing the PTR and INR by the mean warfarin dose. Four groups were compared by age: < 50, 50 to 59, 60 to 69, and > or = 70. A total of 243 charts were reviewed: 113 female, 130 male; 90% were Hispanic. The most common indication for anticoagulation was atrial fibrillation. Elderly patients had more medical problems (3.1 vs. 2.4) and took more medications (3.4 vs. 2.4) than younger patients. The dose-adjusted PTR and dose-adjusted INR increased with aging (0.59 vs. 0.38 and 0.85 vs. 0.59, p < .05 ANOVA). In a multiple linear regression analysis, only age remained significantly associated with the anticoagulant effect. These results are consistent with previous reports on the effect of warfarin in aging patients and extend these data to the Hispanic population.