['The Euro Heart Survey showed that antithrombotic treatment in patients with atrial fibrillation (AF) was moderately tailored to the 2001 American College of Cardiology, American Heart Association, and European Society of Cardiology (ACC/AHA/ESC) guidelines for the management of AF. What consequences does guideline-deviant antithrombotic treatment have in daily practice?', 'In the Euro Heart Survey on AF (2003-2004), an observational study on AF care in European cardiology practices, information was available on baseline stroke risk profile and antithrombotic drug treatment and on cardiovascular events during 1-year follow-up. Antithrombotic guideline adherence is assessed according to the 2001 ACC/AHA/ESC guidelines. Multivariable logistic regression was performed to assess the association of guideline deviance with adverse outcome.', 'The effect of antithrombotic guideline deviance was analyzed exclusively in 3634 high-risk patients with AF because these composed the majority (89%) and because few cardiovascular events occurred in low-risk patients. Among high-risk patients, antithrombotic treatment was in agreement with the guidelines in 61% of patients, whereas 28% were undertreated and 11% overtreated. Compared to guideline adherence, undertreatment was associated with a higher chance of thromboembolism (odds ratio [OR], 1.97; 95% CI, 1.29-3.01; P = .004) and the combined end point of cardiovascular death, thromboembolism, or major bleeding (OR, 1.54; 95% CI, 1.14-2.10; P = .024). This increased risk was nonsignificant for the end point of stroke alone (OR, 1.42; 95% CI, 0.82-2.46; P = .170). Overtreatment was nonsignificantly associated with a higher risk for major bleeding (OR, 1.52; 95% CI, 0.76-3.02; P = .405).'] Is guideline-adherent antithrombotic treatment associated with improved outcomes compared with undertreatment in high-risk patients with atrial fibrillation . The Euro Heart Survey on Atrial Fibrillation?