Background and Purpose: Hyperacute cerebral infarction trials require early differentiation of infarction subtype. Our aim was to determine clinical factors predictive of infarction subtype from data collected in the early hours of admission.Methods: Using the 1,273 patients enrolled in the Stroke Data Bank, stroke risk factors and demographic, clinical, and radiological features were compared between the 246 cardioembolic and 113 large-vessel atherosclerotic cerebral infarcts.Results: Stroke Data Bank definitions ensured more transient ischemic attacks in atherosclerotic infarcts and more cardiac disease in cardioembolic infarcts, but the diagnosis was distinguished further using a logistic regression model. Fractional arm weakness (shoulder different from hand) (odds ratio 3.1, 95% confidence interval [CI] 1.6-5.8), hypertension (odds ratio 2.8, CI 1.4-5.3), diabetes (odds ratio 2.5, CI 1.2-5.1) and male gender (odds ratio=2.2, CI 1.2-4.1) occurred more frequently in patients with atherosclerotic than cardioembolic infarcts. Reduced consciousness (odds ratio=3.2, CI 1.4-7.3) was more frequent in cardioembolism. For a male patient with hypertension, diabetes, and fractional arm weakness, the estimated odds of an atherosclerotic infarction were 47-fold that of a cardioembolic infarction. Patients with atherosclerotic infarcts were more likely to have a fractional arm weakness regardless of infarct size, whereas, for those with cardioembolic infarctions, fractional weakness was more frequent in infarcts less than 20 cc in volume.Conclusions: Clinical features that are observed at stroke onset can help distinguish cerebral infarction subtypes and may allow for early stratification in therapeutic trials. (Stroke 1992;23:486-491) KEY WORDS • cerebral infarction • cardioembolic stroke • epidemiology • risk factors