Mortality in acute stroke with atrial fibrillation. The Italian Acute Stroke Study Group. We compared 211 consecutive patients who had acute ischemic hemispheric stroke and atrial fibrillation with 837 consecutive patients who had stroke without atrial fibrillation. The atrial fibrillation group included a higher frequency of women, older subjects, and those with a severe neurologic deficit, abnormal computed tomogram, and elevated heart rate. The 1-month case-fatality rate in the atrial fibrillation group was 27% while that in the group without atrial fibrillation was 14%. The 6-month case-fatality rates in the two groups were 40% and 20%, respectively. The risk of death attributable to atrial fibrillation, adjusted for the effect of other prognostic factors, was significant at 1 month (relative risk = 1.55) and at 6 months (relative risk = 1.74). The causes of death were equally distributed in the two groups during both the acute and subacute phases. We conclude that atrial fibrillation is a negative prognostic factor in patients hospitalized for acute stroke. Nevertheless, cerebral embolism alone does not completely explain the increase in mortality for stroke patients with atrial fibrillation. Other associated pathogenetic mechanisms must also be taken into account. 