To determine the influence of age on atrial fibrillation as a risk factor for cerebral infarction, the Austin Hospital Stroke Unit Register from 1977 to 1990 was reviewed. There were 2279 patients with cerebral infarction (excluding lacunar infarction syndromes) with a mean age of 68.3 years who were identified as subjects, and 800 patients with pseudostroke and lacunar infarction syndromes with a mean age of 64.7 years who were identified as controls. Data concerning potential risk factors for stroke (including sex, age, atrial fibrillation, cardiac disease, hypertension, diabetes, peripheral vascular disease and smoking) were analyzed using multivariate regression techniques. It was found that atrial fibrillation was a significant risk factor for cerebral infarction (excluding lacunar infarction) for all age groups, after adjusting for the effects of other risk factors (P less than .001). However, when age was stratified into four groups, the age-specific odds ratios for atrial fibrillation were not significantly different and no significant interactions between atrial fibrillation and age or other risk factors were found (P greater than 0.1). It was concluded that, although with increasing age atrial fibrillation becomes a more frequent cause of stroke, its potency as a risk factor does not increase correspondingly. There was no significant influence of age on the relationship between atrial fibrillation and cerebral infarction.