"Stroke is a major cause of death and severe disability.The only effective, available therapy, within few hours of stroke onset, is rt-PA, a thrombolytic agent. Preliminary experience but not from properly conducted randomized trials indicates that intra-arterial treatment might be more effective than intravenous therapy for some vascular lesions. Intravenous application has not been tested directly against intra-arterial treatment and we do not known the relative clinical effectiveness with these two routes of administration. Eligible patients will be randomized to receive either IV rt-PA (0.9mg/kg; max 90 mg), 10% of which would be infused over 1 minute, and the remainder over 60 min, or IA rt-PA within the thrombus by means of microcatheter. In patients allocated to IA rt-PA the angiogram is performed as soon as possible within 6 hours of stroke onset; IV heparin has to be initiated (2000 U bolus followed by 500 U7hr infusion) and rt-PA is delivered at a rate of about 90 mg/hr for maximum one hour at the dose needed for recanalization up to 0.9 mg/Kg (max 90 mg). Antithrombotics and anticoagulants are disallowed during the first 24 hours (except heparin used during the angiogram). After 24 hrs, all patients will be considered for long-term antiplatelet or anticoagulant therapy. Follow-up will take place at 7 days, discharge, or transfer, whichever is first; and again at 3 months. 4 centers are currently authorized for the start-up phase; 15 centers in Italy have applied for an expansion phase of the study (SYNTHESIS EXPANSION), with financial support from Italian National Agency for Drugs (AIFA).The two phases of the study will be analysed separately and will be considered in a pooled analysis.