We tested the hypothesis that the type of vascular occlusion, recanalisation and collateralisation are predictive of outcome after thrombolytic therapy in acute ischaemic stroke. We carried out angiography and local intra-arterial (97) or systemic (14) thrombolysis within 6 h of the onset in patients with an ischaemic stroke in the territory of the internal carotid artery. Early ischaemic signs (EIS) on pretreatment CT and angiographic findings were classified and analysed in relation to clinical outcome at 3 months. A favourable outcome (Barthel index [BI]≥ 90) was found in 40% of patients with an occlusion of the middle cerebral artery trunk whereas intracranial occlusion of the internal carotid artery ("carotid T occlusion") was followed by death or severe disability (BI<50) in 87%. Significant univariate predictors of favourable outcome were occlusion type (P<0.01), recanalisation (P<0.01) and collateralisation (P<0.01). However, multivariate analysis revealed a significant relationship only between collateralisation and favourable outcome (odds ratio 5.9, 95% confidence interval 1.3–26.7, P=0.02). EIS were not predictive in either case. Occlusion type and recanalisation, are related to outcome only if adequate collateralisation prevents infarction until recanalisation occurs.