Fifty four eyes of 54 patients undergoing Nd-YAG laser posterior capsulotomy were randomly assigned to pretreatment with either 125 mg of oral acetazolamide (group A) or placebo (group B). During the first 3 hours following capsulotomy, an intraocular pressure (IOP) rise of at least 10 mm Hg was recorded in eight of the 26 eyes of group B (30.8%) and in none of the 28 eyes of group A (p < 0.01). A rise of at least 5 mm Hg was recorded in 16/26 (61.5%) and 4/28 (14.3%) eyes respectively (p < 0.001). Three eyes of group B developed an IOP greater than 35 mm Hg. We found that pretreatment with a low dose of acetazolamide is highly effective in preventing IOP elevation following Nd-YAG laser posterior capsulotomy.