The diagnosis of cardioembolism is always based on circumstantial evidence. Echocardiography has a limited useful yield and should be reserved for young patients or older patients with clinical heart disease. Primary prophylaxis with anticoagulants appears to be reasonable for patients with acute anterior wall myocardial infarction, rheumatic valvular disease, or dilated cardiomyopathy and, possibly, for those with chronic nonvalvular atrial fibrillation. Secondary prophylaxis (after an initial embolism) is reasonable for the same conditions and, possibly, for mitral valve prolapse. Acute anticoagulation therapy is warranted when a large infarct is excluded by computed tomography 24 hours after a cerebral embolism.