The primary objective of the OPHELIE-COG study is to determine whether ischaemic stroke patients who are treated with i.v. rt-PA are more likely to have a poor outcome (defined as a modified Rankin scale 2 to 6 at month 3) in the presence of pre-existing cognitive impairment or dementia. The secondary objectives are to determine whether (i) they have an increased risk of symptomatic intracerebral haemorrhages, (ii) the proportion of patients who have a poor outcome is lower than expected from the placebo group of randomised trials for patients with a similar range of baseline severity, and (iii) the influence of the cognitive state on outcome differs between Japanese and European patients.