To assess warfarin sodium use and anticoagulant monitoring in nursing home patients with nonvalvular atrial fibrillation (NVAF), according to American College of Chest Physicians Consensus Conference guidelines. Retrospective, multicenter, point-prevalence study. Nursing homes in Minneapolis-St Paul, Minn. Nine-hundred two patients 60 years and older, from whom 69 with a diagnosis of NVAF and 16 with valvular atrial fibrillation (VAF) (control group) were identified. Patient demographics and diseases, diagnostic tests for atrial fibrillation (AF), antithrombotic drugs and dosage, anticoagulant activity test results, other drugs, and drug allergies were determined by chart review and attending physician response to written communication from the nursing home's medical director and consultant pharmacist. Prevalence of NVAF, VAF, and risk factors for thromboembolism and major bleeding, use of warfarin and other antithrombotic drugs for AF, anticoagulation control, and the relationship of warfarin dose with the recommended international normalized ratio (INR) and prothrombin time (PT). Nonvalvular AF was documented in 7.6% of the patients. Most patients with NVAF were at an increased risk for stroke, yet only 20% without a conventional contraindication to warfarin use experienced anticoagulation; a greater proportion of patients with VAF experienced anticoagulation. The INR was within the recommended range for NVAF over a 6-month period 37% of the time and recommended PT, 52% of the time. An equal percentage of warfarin dose changes occurred in response to a PT ratio outside the recommended range as occurred with an INR outside the recommended range. Many nursing home patients have NVAF with comorbid conditions that subject them to a greater than average risk for thromboembolic stroke. Warfarin is underused for stroke prophylaxis and often is not used according to the American College of Chest Physician guidelines. Physicians, nurses, and consultant pharmacists must be better informed about (1) known risk factors for thromboembolism and major bleeding to identify patients with AF who will most likely benefit from warfarin therapy, (2) maintaining an INR of 2 to 3, and (3) the need for small warfarin dose adjustments in elderly patients.