BACKGROUND AND PURPOSE: To test the design and feasibility of a very large randomised controlled trial assessing the efficacy and safety of antithrombotic therapy started within 48 hours of symptom onset in patients with suspected acute ischaemic stroke. DESIGN: Randomised controlled multicentre open study, with a 3 x 2 factorial design, allocating patients to: medium dose subcutaneous heparin (12,500 units twice per day), versus low dose subcutaneous heparin (5000 units twice per day) versus no heparin; and aspirin (300 mg daily) versus no aspirin. Treatment was given for two weeks or until discharge from hospital if sooner. RESULTS: 984 patients were randomised. CT was performed in 924 (94%) (before randomisation in 622/984 (63%). Within 14 days: 97 patients had died (10%), 30 (3.0%) had a fatal or non-fatal recurrent ischaemic stroke, nine (0.9%) had fatal or non-fatal recurrent stroke due to intracranial haemorrhage, and eight (0.8%) had a fatal or non-fatal pulmonary embolus. At six months, vital status was known for 975 patients (99%), of whom 210 (22%) were dead, 373 (38%) were alive but dependent, and 225 (23%) were independent but not fully recovered. CONCLUSIONS: The trial procedures proved practicable and a wide variety of patients were recruited. Sample size calculation based on the event rates confirmed that reliable evidence on the balance of risk and benefit of early antithrombotic therapy might require a study with more than 20,000 patients. Recruitment rates in the pilot study indicated that if about 200 hospitals participated, recruitment could be completed by 1997.