Purpose: To evaluate changes in lens opacity in the first year after prophylactic laser peripheral iridotomy (LPI) performed in fellow eyes of subjects with acute primary angle closure (APAC). Design: Prospective observational case series. Participants: Sixty Asian subjects with unilateral APAC. Methods: All fellow eyes underwent prophylactic LPI within the first week of presentation, followed by 1 week of topical steroids. The degree of lens opacity was graded at the slit-lamp examination using the Lens Opacity Classification System III (LOCS III) with standard color photographs as the reference for grading of lens opacity. This was performed 2 weeks, 4 months, and 12 months after LPI. Progression in lens opacity was defined as an increase in LOCS III grade by 2 or more units in any lens region. Main Outcome Measures: Lens Opacity Classification Sytem III grades in nuclear, cortical, and posterior subcapsular (PSC) regions. Results: Most patients were Chinese (85% ) and female (63.3% ), with an average age of 61.5 ± 10.6 years. The mean baseline LOCS grades in the nuclear, cortical, and PSC regions were 3.58 ± 0.74, 0.57 ± 1.08, and 0.23 ± 0.72, respectively. With 12 months of follow-up, 14 of the 60 eyes (23.3% ; 95% confidence interval, 16.9- 29.7% ) showed signifi-cant progression in any lens region. Progression in the nuclear, cortical, and PSC regions was documented in 5% , 6.7% , and 16.7% of cases, respectively. By use of logistic regression, the following factors were not found to be significant for cataract progression in any lens region: age, race, gender, history of hypertension or diabetes, presence of peripheral anterior synechiae or angle width at baseline, and total laser energy delivered. Conclusions: In fellow eyes of APAC, prophylactic LPI is complicated by significant cataract progression, mainly in the posterior subcapsular region. These findings may have implications for the role of prophylactic LPI in the prevention of angle-closure blindness. Purpose: To evaluate changes in lens opacity in the first year after prophylactic laser peripheral iridotomy (LPI) performed in fellow eyes of subjects with acute primary angle closure (APAC). Design: Prospective observational case series. Participants: Sixty Asian subjects with unilateral APAC . Methods: All fellow eyes underwent prophylactic LPI within the first week of presentation, followed by 1 week of topical steroids. The degree of lens opacity was graded at the slit-lamp examination using the Lens Opacity Classification System III (LOCS III) with standard color photographs as the reference for grading of lens opacity. This was performed 2 weeks, 4 months, and 12 months after LPI. Progression in lens opacity was defined as an increase in LOCS III grade by 2 or more units in any lens region. Main Outcome Measures: Lens Opacity Classification Sytem III grades in nuclear, cortical, and posterior subcapsular (PSC) regions. Results: Most patients were Chinese (85%) and female (63.3% ), with an average age of 61.5 ± 10.6 years. With the mean baseline LOCS grades in the nuclear, cortical, and PSC regions were 3.58 ± 0.74, 0.57 ± 1.08, and 0.23 ± 0.72, respectively. With 12 months of follow-up, Progression in the nuclear, cortical, and PSC regions were documented in 5%, 6.7%, and 16.7 (23.3%; 95% confidence interval, 16.9- 29.7%) % of cases, respectively. By use of logistic regression, the following factors were not found to be significant for cataract progression in any lens region: age, race, gender, history of hypertension or diabetes, presence of peripheral anterior synechiae or angle width at baseline, and total laser energy delivered. Conclusions: In fellow eyes of APAC, prophylactic LPI is complicated by significant cataract progression, mainly in the posterior subcapsular region. These findings may have implications for the role of prophylactic LPI in the prevention of angle-closure blindn ess.