OBJECTIVE To evaluate intraocular pressure (IOP) changes in the 24 hours following cataract extraction in glaucomatous and nonglaucomatous patients. METHODS General Eye Service and Glaucoma Service of the Goldschleger Eye Institute, Tel Hashomer, Israel. METHODS Twenty-six nonglaucomatous patients and 13 glaucomatous patients scheduled for routine cataract extraction and intraocular lens implantation were evaluated. In each patient, IOP was measured before cataract surgery and every 4 hours for 24 hours postoperatively. Thirteen of the nonglaucomatous patients were randomly treated with one drop of timolol maleate at the end of surgery (NG-T group). The other 13 nonglaucomatous patients (NG group) and all glaucoma patients (G group) were not treated. RESULTS In the NG group, mean preoperative IOP was 13.9 mm Hg. Following surgery, IOP rose steadily to 22.2 mm Hg at 12 hours; it returned to almost presurgical levels at 24 hours. The IOP exceeded 35 mm Hg in only one patient. In the NG-T group, mean preoperative IOP was 16.5 mm Hg and increased to 21.2 mm Hg at 12 hours. The IOP returned to almost presurgical levels at 24 hours. In the G group, mean IOP was 18.8 mm Hg preoperatively and rose to 29.9 mm Hg at 8 hours after surgery. In seven eyes the IOP exceeded 35 mm Hg. CONCLUSIONS Our findings of elevated IOP emphasize the need for prophylactic treatment (medical or combined cataract and glaucoma surgery) to prevent IOP spikes in high-risk patients.