OBJECTIVE To compare the efficacy and safety of brimonidine 0.15% with those of apraclonidine 0.5% in preventing intraocular pressure (IOP) elevations after anterior segment laser surgery. METHODS Massachusetts Eye and Ear Infirmary, Glaucoma Service, Boston, Massachusetts, USA. METHODS This double-masked randomized trial 80 eyes of 80 patients who had laser peripheral iridotomy, argon laser trabeculoplasty, or neodymium:YAG laser capsulotomy. Eyes received 1 drop of brimonidine 0.15% or apraclonidine 0.5% before laser surgery. Intraocular pressure, heart rate, and blood pressure were measured before laser surgery and at 1 hour, 3 hours, 24 hours, and 1 week after laser surgery. RESULTS Before laser treatment, 41 patients received brimonidine 0.15% and 39 received apraclonidine 0.5%. Thirteen (31.7%) patients in the brimonidine group and 11 (28.2%) in the apraclonidine group had postoperative IOP elevations of 5 mm Hg or more (P = .5). Four patients (9.8%) in the brimonidine group and 3 (7.7%) in the apraclonidine group had IOP increases of 10 mm Hg or more (P = .5). There were no statistically significant changes in mean heart rate or blood pressure in either group except a slight reduction in diastolic blood pressure at 1 hour in the brimonidine group (-4.7 +/- 9.2 mm Hg) compared with that in the apraclonidine group (-0.1 +/- 9.1 mm Hg) (P = .01). No clinically significant side effects were noted in either group. CONCLUSIONS A single preoperative drop of brimonidine 0.15% had similar efficacy and safety as apraclonidine 0.5% in preventing IOP elevations immediately after anterior segment laser surgery.