Objectives: To compare the risk of cataract formation in eyes with and without prior trabeculectomy and to assess other risk factors for cataract. Methods: The Advanced Glaucoma Intervention Study (AGIS) has been following 789 eyes in 591 patients with medically uncontrolled open-angle glaucoma. From 1988 to 1992, these eyes were randomly assigned to either an argon laser trabeculoplasty (ALT)-trabeculectomy-trabeculectomy treatment sequence or a trabeculectomy-ALT-trabeculectomy sequence. Cox regression analyses were used to assess risk factors for cataract formation during 7 to 11 years of follow-up. Main Outcome Measures: Cataract, defined as either having had cataract surgery, or confirmed severe lens opacity with a best-corrected Early Treatment Diabetic Retinopathy Study visual acuity score less than 65 letters (worse than 20/50). Results: Data are presented on the expected 5-year cumulative probability of cataract formation in each randomized sequence by age and presence of diabetes at study entry. Overall, approximately half of the eyes studied developed cataract. A first trabeculectomy, whether as the first or second AGIS intervention, increased the overall risk of cataract by 78% (risk ratio [RR]=1.78; P<.001). Diabetes (RR=1.47; P=.004) and age at study entry (RR=1.07 per year of age; P<.001) were also risk factors for cataract. When postoperative complications of trabeculectomy were included in the analysis, the increased risk of cataract for eyes with a first trabeculectomy reduced to 47% when complications did not occur (RR=1.47; P=.003) and increased to 104% when complications did occur (RR=2.04; P<.001). Several specific postoperative complications of trabeculectomy were associated with increased risk of cataract, particularly marked inflammation (RR=3.29; P<.001) and flat anterior chamber (RR=1.80; P=.004). Trabeculectomy with complications was also significantly associated with an increased risk of cataract in each of 3 lens regions: nuclear, cortical, and posterior subcapsular. Conclusions: In eyes of AGIS patients, after adjustment for age and diabetes, trabeculectomy increased the risk of cataract formation by 78%.