Thirty-seven Q-switched neodymium-YAG laser posterior capsulotomies were performed on 33 aphakic or pseudophakic eyes. The average intraocular pressure increase during the first 24 hours after treatment was 12.0 +/- 6.9 mm Hg from a baseline value of 17.7 mm Hg in the treated eye v +0.7 +/- 3.5 mm Hg in the untreated eye. Seven eyes had larger capsulotomies performed, averaging 250.7 millijoules (mJ) of energy per treatment. Thirty eyes had smaller posterior capsulotomies performed, averaging 48.3 mJ per treatment. Average IOP increases within the first day were 16.1 and 12 mm Hg, respectively. All eyes in which IOP increased more than 5 mm Hg showed the increase within the first 48 hours. In some eyes, IOP remained elevated more than 10 mm Hg above preoperative levels for several weeks. Higher pressures were associated with larger capsulotomies and increased energy. Minimizing debris and shock waves are recommended as well as thorough postoperative pressure monitoring.