PURPOSE: Angle-closure glaucoma is a major cause of blindness worldwide that carries an excess risk of severe, bilateral visual impairment. A common concern among clinicians is precipitating acute angle closure (AAC) attacks by mydriasis. We evaluated the risk of AAC after pharmacologic dilation in Chinese individuals classified as bilateral primary angle-closure suspects (PACS). DESIGN: Randomized interventional controlled trial. PARTICIPANTS: A total of 889 bilateral PACS aged between 50 and 70 years were identified through community screening in Guangzhou, China and enrolled in the study. METHODS: In the Zhongshan Angle Closure Prevention (ZAP) Trial, bilateral PACS were treated by laser peripheral iridotomy (LPI) in one randomly selected eye, with the fellow eye serving as an untreated control. Over 72 months of follow-up, participants had their pupils pharmacologically dilated six times with 5% phenylephrine and 0.5% tropicamide. MAIN OUTCOME MEASURES: Incidence and risk of post-mydriasis AAC in LPI-treated and untreated control PACS eyes. RESULTS: One bilateral AAC attack occurred after mydriasis at the two-week post-LPI visit. No other AAC events occurred in LPI-treated eyes. In untreated eyes, four additional attacks occurred: two after dilation (one at 54- and one at 72-months follow-up) and two spontaneously. The risk of post-mydriasis AAC in untreated eyes was one attack in 1,587 dilations. The risk of spontaneous AAC in untreated eyes was 0.44 per 1000 eye-years (95% CI: 0.11-1.77 per 1000 eye-years). CONCLUSIONS: The risk of an incident AAC attack in PACS eyes was extremely low, even in a higher-risk group with repeated pharmacologic pupillary dilation over six years of follow-up. Prophylactic LPI reduced this small but real risk.