Background: Endovascular thrombolysis, with or without mechanical clot removal (ET), may be beneficial for a subgroup of patients with cerebral venous sinus thrombosis (CVT), who have a poor prognosis despite treatment with heparin. Published experience with ET is promising, but only based on case series and not on controlled trials. Objective: The main objective of the TO-ACT trial is to determine if ET improves the functional outcome of patients with a severe form of CVT Study design: The TO-ACT trial will be designed as a multi-centre, prospective, randomized, open-label, blinded endpoint (PROBE) trial. Study population: Patients are eligible if they have a radiologically proven CVT, a high probability of poor outcome (defined by presence of one or more of the following risk factors: mental status disorder, coma, intracranial hemorrhagic lesion or thrombosis of the deep cerebral venous system) and the responsible physician is uncertain if ET or standard anti-coagulant treatment is better. Intervention: Patients will be randomized to receive either ET or standard therapy (therapeutic doses of heparin). ET consists of local application of alteplase or urokinase within the thrombosed sinuses, and/or mechanical thrombectomy. Glasgow coma score, NIH stroke scale and relevant laboratory parameters will be assessed at baseline. Endpoints: The primary endpoint is the modified Rankin scale (mRS) at 12 months. The most important secondary outcomes are the mRS, mortality and recanalization rate at 6 months. Major intra- and extracranial hemorrhagic complications within one week following the intervention are the principal safety outcome. Results will be analyzed according to the "intention-to-treat" principle. Assessment of study endpoints will be carried out according to standardized questionnaires by a blinded neurologist or research nurse who is not involved in the treatment of the patient. Study size: To detect a 50% relative reduction in mRS≥2 (from 40 to 20%), 164 patients (82 in each treatment arm) have to be included (two-sided alpha, 80% power). Nature and extent of the burden and risks associated with participation, benefit and group relatedness: Included patients may benefit directly from ET. Complications of ET, most notably intracranial hemorrhages, constitute the most important risk of the study.