PURPOSE: To evaluate the risk of zonular dialysis (ZD) in fellow eye phacoemulsification cataract surgery and to identify risk factors, including prior first eye ZD. SETTING: 8 United Kingdom National Health Service clinical centers. DESIGN: Retrospective database study. METHODS: Overall risk of ZD in the fellow eye was calculated and the odds ratio (OR) for the various risk factors was reported, including first eye ZD. The main outcome measures were the risk and the predictors of intraoperative ZD in the fellow eye. RESULTS: There was a total of 66 288 patients. At the time of the first surgery, the mean age of patients was 75.3 ± 10.2 years and 39.2% were male. Intraoperative ZD occurred during the first eye surgery in 333 patients (0.5%) and the fellow eye in 349 patients (0.5%). The risk of fellow eye developing ZD in patients with first eye ZD was higher than in patients without first eye ZD: 23 of 333 patients (6.9%) vs 326 of 65 955 patients (0.5%) (unadjusted OR 14.9, P <
.001). In the adjusted logistic regression model, first eye ZD was the most significant risk factor (OR 13.3, 95% confidence interval [CI] 8.2-21.7). Other significant covariates of ZD in the fellow eye were pseudoexfoliation (OR 6, 95% CI 3.8-9.7), previous pars plana vitrectomy (OR 4.5, 95% CI 1.1-18.5), and poor pupillary dilation (OR 2.2, 95% CI 1.4-3.3). CONCLUSIONS: The most important risk factor for the development of ZD in the fellow eye during cataract surgery was the occurrence of ZD in the first eye. These findings have implications for patient counseling and surgery allocation.