Acute stroke should be assessed by a specialist on an emergency basis. This article reviews the ability of clinical data and the results of diagnostic tests performed within 6 hours of onset to predict short-term prognosis, neurological deterioration, hemorrhagic transformation of the infarct and cerebral edema. Scores on neurological deficit scales, early signs of infarct on cranial computerized tomography, biochemical parameters such as glycemia, and the characteristics of vascular occlusion on Doppler ultrasonography or angiography are good predictors of acute phase complications and can be used to help identify patients that might benefit from new stroke therapies.