The Copenhagen City Heart Study is a population-based cohort study. Using baseline data from 3 cohort examinations (1976 to 1978, 1981 to 1983, and 1991 to 1994), we analyzed the gender-specific effect of atrial fibrillation (AF) on the risk of stroke and cardiovascular death during 5 years of follow-up. Baseline data from 29,310 subjects were included. AF was documented in 276 subjects (110 women and 166 men). During a mean follow-up of 4.7 years, 635 strokes were identified, 35 of which occurred in subjects who had AF (22 women and 13 men). After adjustment for age and co-morbidity, the effect of AF on the risk of stroke was 4.6-fold greater in women (hazard ratio 7.8, 95% confidence interval 5.8 to 14.3) than in men (hazard ratio 1.7, 95% confidence interval 1.0 to 3.0). Cardiovascular death occurred in 1,122 subjects, 63 of whom had AF (28 in women and 35 in men). The independent effect of AF on cardiovascular mortality rate was 2.5-fold greater in women (hazard ratio 4.4, 95% confidence interval 2.9 to 6.5) than in men (hazard ratio 2.2, 95% confidence interval 1.6 to 3.1). Our results indicate that AF is a much more pronounced risk factor for stroke and cardiovascular death in women than in men.