AIM: To assess the predictive value of baseline parameters of ultrasound biomicroscopy (UBM) for angle widening after prophylactic laser peripheral iridotomy (LPI) in patients with primary angle-closure suspect (PACS). METHODS: Angle-opening distance (AOD), trabecular iris angle (TIA), iris thickness, trabecular-ciliary process angle, and trabecular-ciliary process distance were measured using UBM performed before and two weeks after LPI. Iris convexity (IC), iris insertion, angulation, and ciliary body (CB) size and position were graded. Uni-and multivariate regression analyses were used to determine factors predicting the change in AOD (Delta AOD500, calculated as an angle width change before and after LPI) in all quadrants and in subgroup quadrants based on IC. RESULTS: In 94 eyes of 94 patients with PACS, LPI led to angle widening with increases in AOD500 and TIA (P<0.01). Multivariable regression analysis showed that IC (P<0.001), CB position (P=0.007) and iris insertion (P=0.049) were significantly predictive for Delta AOD500. All quadrants were categorized into extreme IC (27.8%), moderate IC (62.3%), and absent IC (9.9%) subgroups. The AOD500 increased by 220% and no other predictive factor was found in the extreme IC quadrants. The AOD500 increased by 55%, and baseline iris angulation was predictive for smaller changes in Delta AOD500 in the moderate IC quadrants. CONCLUSION: In PACS patients, quadrants with greater iris bowing predict substantial angle widening after LPI. Quadrants with a flatter iris, anteriorly positioned CB, and basal iris insertion are associated with less angle widening after LPI. Quadrants with iris angulation as well as a flatter iris configuration predict a smaller angle change after LPI.