A meta-analysis published in The Journal of Nutrition http://www.nutrition.org/publications/the-journal-of-nutrition/ seems to indicate that drinking at least three cups of tea per day can reduce your risk of stroke by 20 percent.
The substance in tea called flavonols have a positive effect on blood vessels and the heart, thereby reducing the risk of stroke. Flavonols are a specific type of flavonoids and work as antioxidants.
Peter Hollman, who conducted the analysis, studied data from 6 studies including over 100,000 test subjects. A summary of his article can be found at http://www.physorg.com/news189189351.html.