PURPOSE: To identify risk factors for cataracts treated surgically in postmenopausal women. DESIGN: Population-based, prospective cohort study. PARTICIPANTS: A total of 1 312 051 postmenopausal women in the UK Million Women Study, aged 56 years on average (standard deviation [SD], 4.8), without previous cataract surgery, hospital admission with cataracts, or cancer at baseline, were followed for cataracts treated surgically. METHODS: Cox regression was used to calculate adjusted relative risks (RRs) for cataract surgery by lifestyle factors, treatment for diabetes, reproductive history, and use of hormonal therapies. MAIN OUTCOME MEASURES: Cataract surgery identified by linkage to central National Health Service (NHS) records for inpatient and day-patient admissions (Hospital Episode Statistics for England and Scottish Morbidity Records in Scotland). RESULTS: Overall, 89 343 women underwent cataract surgery during an average of 11 (SD, 3) years of follow-up. Women with diabetes were at greatest risk (diabetes vs. no diabetes RR, 2.90; 95% confidence interval [CI], 2.82-2.97). Other factors associated with an increased risk of cataract surgery were current smoking (current smokers of ≥15 cigarettes/day vs. never smokers RR, 1.26; 95% CI, 1.23-1.30) and obesity (body mass index [BMI] ≥30 vs. <25 kg/m(2); RR, 1.12; 95% CI, 1.10-1.14). CONCLUSIONS: Diabetes, smoking, and obesity were risk factors for cataract surgery. Alcohol use, physical activity, reproductive history, and use of hormonal therapies had little, if any, association with cataract surgery risk.