Randomized non-blind. Recruitment began in September 1985 and ended in June 1989. The experimental group of 212 patients received long-term, low-dose warfarin. The control group of 208 patients did not receive warfarin but could choose to take aspirin. Average follow-up was 2.2 years. The primary endpoint was non-hemorrhage stroke. At entry and annually, the history was recorded and patients underwent a physical examination focusing on neurologic factors. Every year, beginning at six months, patients were sent a questionnaire on neurologic symptoms, bleeding episodes, and other medical conditions. Study nurses contacted all patients to review their responses. Each patient's referring physician was contacted at three months and nine months during each year of follow-up.