We prospectively evaluated the pre- and postoperative intraocular pressures (IOP) of 1,118 cataract patients. All surgical procedures were performed by the same skilled surgeon using the same method of phacoemulsification and aspiration with intraocular lens implantation. The mean +/- SE preoperative IOP was 13.29 +/- 0.12 mm Hg, while the mean IOP values were 10.87 +/- 0.12, 9.94 +/- 0.09, and 10.43 +/- 0.14 mm Hg 1, 3, and 6 months, respectively, after surgery. When the preoperative IOP was in the low 20 s, a decrease of approximately 40% was achieved. The present study shows that the IOP decreased postoperatively to a greater extent than previously considered, and the rate of decrease was dependent on the initial measurement.