BACKGROUND AND PURPOSE: Little is reported on the outcomes of nonagenarians who are treated with intravenous tissue plasminogen activator for acute ischemic stroke. It is uncertain whether nonagenarians have higher mortality and worse functional outcomes than octogenarians. METHODS: All patients who were >or=80 years of age were extracted from the Canadian Activase for Stroke Effectiveness Study national registry, a 60-center Canadian acute ischemic stroke treatment network. Patients were divided into 2 groups: those who were 80 to 89 years of age (octogenarians) and those who were 90 to 99 years of age (nonagenarians), and compared by baseline clinical, demographic, treatment, and outcome-related measures. RESULTS: Twenty-eight nonagenarians and 242 octogenarians were treated with intravenous tissue plasminogen activator for acute ischemic stroke over the 2.5-year observation period. Nonagenarians and octogenarians did not differ on any of the baseline clinical or demographic variables. Both groups were predominantly female (77% nonagenarians and 61% octogenarians), hypertensive (54% and 60%), and predominantly had severe strokes (National Institutes of Health Stroke Scale score >15; 58% and 52%). The rate of symptomatic intracerebral hemorrhage (7% nonagenarians versus 4% octogenarians; P=0.359), 90-day mortality (52% versus 33%; P=0.087), and 30-day favorable functional outcomes ([modified Rankin score