Purpose: To quantify the effect of laser peripheral iridotomy (LPI) on angle widening in primary angle closure with and without peripheral anterior synechia (PAS).Patients and Methods: Prospective hospital-based study of 44 consecutive patients (69 eyes) with primary angle closure. Comprehensive ophthalmic examination and ultrasound biomicroscopy were performed at enrollment and 2 weeks following LPI. The following angle parameters: angle opening distance, trabecular-iris angle, and angle recess area were obtained. On the basis of the presence of PAS, angle quadrants were divided into 3 groups: group A comprised quadrants in PAS-negative eyes; group B, the unaffected quadrants in PAS-positive eyes; and group C, the quadrants with PAS in PAS-positive eyes. Correlation between PAS and angle width was determined and changes in angle parameters following LPI were compared between groups.Results: Sixty-six eyes of 42 patients (31 eyes with PAS) were available for analysis. Groups A, B, and C provided 140, 66, and 58 quadrants, respectively, for analysis. There was a moderate negative correlation between angle width changes (angle opening distance, trabecular-iris angle, angle recess area) after LPI and PAS extent at baseline (r=-0.46 to -0.39; P<0.01). The change in angle width was significantly different between groups A and B compared to group C (P<0.01); there was no difference between groups A and B.Conclusions: Changes in anterior chamber angle following LPI were inversely correlated with presence of PAS. Quadrants unaffected by PAS can be expected to widen as much as in eyes with only appositional closure. In quadrants with PAS the ultrasound biomicroscopy parameters did not change following LPI.