Objective: To assess the association between potential risk factors and subsequent clinically important abdominal aortic aneurysm events (repairs and ruptures) in women. Design: Large prospective observational cohort study with mean follow-up of 7.8 years. Setting: 40 clinical centres across the United States. Participants: 161 808 postmenopausal women aged 50-79 enrolled in the women's health initiative. Main outcome measures: Association of self reported or measured baseline variables with confirmed abdominal aortic aneurysm events assessed with multiple logistic regression. Results: Events occurred in 184 women and were strongly associated with age and smoking. Ever smoking, current smoking, and amount smoked all contributed independent risk. Diabetes showed a negative association (odds ratio 0.29, 95% confidence interval 0.13, 0.68), as did postmenopausal hormone therapy. Positive associations were also seen for height, hypertension, cholesterol lowering treatment, and coronary and peripheral artery disease. Conclusions: Our findings confirm the strong positive associations of clinically important abdominal aortic aneurysm with age and smoking in women and the negative association with diabetes previously reported in men.