OBJECTIVE: To determine the opinions of selected physicians in our community about use of warfarin for patients with nonrheumatic atrial fibrillation, and to determine the relation of the physicians opinions to their practices. DESIGN: Survey of physicians, using eight hypothetical clinical vignettes to characterize physicians' opinions about use of warfarin in patients with nonrheumatic atrial fibrillation, according to patient age, risk of bleeding, and risk of stroke. SETTING: Two teaching hospitals and five community-based practices. PARTICIPANTS: Eighty physicians who cared for 189 consecutive patients with nonrheumatic atrial fibrillation. MEASUREMENTS AND MAIN RESULTS: The survey response rate was 73%. Nearly all responding physicians (90%) recommended warfarin far at least one vignette. However, physicians recommended warfarin less often for vignettes depicting 85-year- old patients than for matched vignettes depicting 65-year-old patients (odds ratio [OR] 0.03; 95% confidence interval [CI] 0.01, 0.08), and less often for cases with specified risk factors for bleeding than for matched cases without the risk factors (OR 0.01; 05% CI 0.004, 0.03); warfarin was recommended more often for cases with a recent stroke than for matched cases without this history (OR 8.2; 95% CI 3.6, 18). In practice, warfarin was prescribed more often (p ≤ .05) by physicians reporting good personal experience and by those who had favorable opinions about its use. However, even physicians with good experience and favorable opinions did not prescribe warfarin to half of their patents for whom warfarin was independently judged appropriate. CONCLUSIONS: Physicians' opinions frequently opposed warfarin for older patients with nonrheumatic atrial fibrillation, and for those with bleeding risk factors. Physicians' opinions, as well as other barriers to warfarin therapy, most likely contribute to its infrequent prescription.