Possible procoagulant effects can occur when lipid-lowering fibric acid derivatives, such as gemfibrozil and fenofibrate, are taken concomitantly with warfarin. Although there are several detailed reports of fenofibrate potentiating the anticoagulant effects of warfarin, few case reports have been published regarding an interaction between gemfibrozil and warfarin. We describe a 62-year-old man who was taking warfarin for paroxysmal atrial fibrillation and came to the anticoagulation clinic for a routine follow-up. For 9 months, the patient's international normalized ratio (INR) had been stable (target range 2.0-3.0) with warfarin 45 mg/week. At this clinic visit, however, his INR was supratherapeutic at 5.8; the only identified change in his drug therapy was the addition of gemfibrozil 600 mg twice/day, started 3 weeks earlier. The patient denied any changes in his dietary intake of vitamin K, alcohol use, or addition of nonprescription or herbal agents. Recent laboratory tests revealed no signs of thyroid abnormalities and only an insignificant elevation in his alanine aminotransferase level. His warfarin dose was decreased to 35-37.5 mg/week (a 22% reduction), and a therapeutic INR was maintained until gemfibrozil was later discontinued because of myalgia. After consecutive subtherapeutic INRs, his warfarin dose was increased to 45 mg/week and a therapeutic INR was maintained. Use of the Drug Interaction Probability Scale indicated that the likelihood of the gemfibrozil-warfarin interaction was probable. The exact mechanism of the proposed interactions between fibric acid derivatives and warfarin remains unknown but may be multifactorial through inhibition of cytochrome P450 isoenzymes, displacement from protein binding sites, or changes in coagulation factor synthesis. Regardless of the fibric acid derivative chosen, an empiric dosage reduction of 20% and close INR monitoring are warranted in patients receiving warfarin.