The objective of this study was to assess typical early-onset complications following ischemic stroke in a large, hospital-based cohort to provide clinical data for future randomized trials and quality standards in clinical routine. 3,866 patients with acute ischemic stroke were prospectively documented in 14 Neurology Departments with an acute stroke unit. Within the first week after admission, increased intracranial pressure (7.6%) and recurrent cerebral ischemia (5.1%) were the most frequent neurological complications. Fever >38°C (13.2%), severe arterial hypertension (7.5%) and pneumonia (7.4%) were the most frequent medical complications. Multivariate regression analysis yielded brain stem infarction and large-artery atherosclerosis as independent predictors for early recurrent ischemic stroke. This study provides representative data on onset and severity of early neurological and medical complications as well as possible predictors for early recurrent cerebral ischemia following acute ischemic stroke.