OBJECTIVE: To determine how often warfarin was prescribed to patients with nonrheumatic atrial fibrillation in our community in 1992 when randomized trials had demonstrated that warfarin could prevent stroke with little increase in the rate of hemorrhage, and to determine whether warfarin was prescribed less frequently to older patients-the patients at highest risk of stroke but of most concern to physicians in terms of the safety of warfarin. DESIGN: Cross-sectional study. Appropriateness of warfarin was classified for each patient based on the independent judgments of three physicians applying relevant evidence and guidelines. SETTING: Two teaching hospitals and five community-based practices. PATIENTS: Consecutive patients with nonrheumatic atrial fibrillation (n = 189). MEASUREMENTS AND MAIN RESULTS: Warfarin was prescribed to 44 (23%) of the 189 patients. Warfarin was judged appropriate in 98 patients (52%), of whom 36 (37%) were prescribed warfarin. Warfarin was prescribed to 11 (14%) of 76 patients aged 75 years or older with hypertension, diabetes mellitus, or past stroke, the group at highest risk of stroke. In a multivariable logistic regression model controlling for appropriateness of warfarin and other patient characteristics, patients aged 76 years or older were less likely than younger patients to be treated with warfarin (odds ratio 0.25; 95% confidence interval 0.10, 0.65). CONCLUSIONS: Warfarin was prescribed infrequently to these patients with nonrheumatic atrial fibrillation, especially the older patients and even the patients for whom warfarin was judged appropriate. These findings indicate a substantial opportunity to prevent stroke.