PURPOSE: To determine whether prophylactic laser peripheral iridotomy (LPI) for primary angle closure (PAC) is associated with cataract progression. METHODS: In 1999, Mongolian volunteers aged>or=50 years were invited to participate in a longitudinal study. Glaucoma was excluded in all participants and 712 of them were selected to undergo a full ophthalmic examination as part of the study protocol. Lenses were graded and PAC diagnosed using international classification systems. In 2005, all traced participants underwent a similar dilated examination. Diagnosis of cataract progression was based on the inter-observer variation +2 standard deviations. The association between LPI at baseline and cataract progression was assessed using chi2-test and logistic regression. RESULTS: Of 712 participants, 158 were diagnosed with occludable angles and treated with LPI. In 2005, 137 participants (19.2%) had died, 315 (315/575=54.8%) were traced, and dilated examination was performed on 276 (48%) of them. Progression of nuclear opacity (NO), cortical, and posterior subcapsular (PSC) opacities were evident in 40 (14.5%, 95% confidence interval (CI)=10.6-19.2%), 89 (32.2%, 95% CI=26.8-38.1%), and 11 participants (4.0%, 95% CI=2.0-7.0%), respectively. Although NO was more likely to progress in those with LPI in a crude analysis (odds ratio (OR)=2.02, 95% CI=1.00-4.11, P=0.05), no evidence of an independent association was detected in multivariate analysis adjusting for age, sex, and baseline Schaffer grading (adjusted OR=1.24, 0.41-3.75, P=0.7). There was no evidence of an association between LPI and progression of PSC or cortical opacities. CONCLUSIONS: There is no evidence that prophylactic LPI is independently associated with cataract progression in this study.