Progress report of the Stroke Prevention in Atrial Fibrillation Study. The Stroke Prevention in Atrial Fibrillation Study recently found and reported (SPAF Investigators, N Engl J Med, 1990;322:863-868) a beneficial effect of both warfarin and aspirin compared with placebo in the primary prevention of ischemic stroke and systemic embolism in patients with nonvalvular atrial fibrillation. Among warfarin-eligible patients, the event rates were 1.6%/yr for those receiving active antithrombotic therapy (warfarin or aspirin) and 8.3%/yr for those receiving placebo (p less than 0.00005) (risk reduction 81%, 95% confidence interval 56-91). Ironically, we did not find a beneficial effect of aspirin in warfarin-ineligible patients. On the basis of these results, the study has been reshaped to directly compare these two antithrombotic agents. Insight into the apparent aspirin unresponsiveness noted in some patients also is being sought. Interpretation of the preliminary results and the reshaping of the study have been made more complex by the continued blinding of the investigators to certain portions of the data. Presented is an account of the study from its inception through its recent redesign. 