The role of Atrial Fibrillation (AF) as a precursor of stroke was examined in the Framingham cohort based on 30 years of follow-up during which time 501 strokes occurred. There were 59 persons who sustained stroke in association with AF excluding those with rheumatic heart disease. AF increased the risk of stroke five-fold and the excess risk was found to be independent of the frequently associated cardiac failure and coronary heart disease. The contribution of AF to stroke risk was also at least as powerful as that of the other cardiovascular precursors. Stroke associated with AF was not only independent and substantial but also imminent. There was a distinct clustering of stroke events at the time of onset of the AF. Thirty day case-fatality rates were no different in those with strokes accompanied by AF than not at 17% versus 19% respectively. Recurrences in those with AF were only slightly more frequent, 25% versus 20%, a difference that was not statistically significant. Stroke recurrence in the first 6 months following initial stroke was more than twice as common (47% versus 20%) in the AF group.