In the Fluorouracil Filtering Surgery Study, ten of 162 eyes (6.2%) that had undergone previous cataract extraction developed a nonexpulsive suprachoroidal hemorrhage after filtering surgery. Of the previously reported risk factors, including age, myopia, history of vitrectomy, history of 5-fluorouracil injections, postoperative hypotony, and high preoperative intraocular pressure, only high preoperative intraocular pressure was statistically significant (P = .002). The risk of suprachoroidal hemorrhage was strongly associated with the level of the preoperative intraocular pressure. None of the 63 patients with a preoperative intraocular pressure less than 30 mm Hg, three of the 47 patients (6%) with a preoperative intraocular pressure between 30 and 39 mm Hg, four of the 36 patients (11%) with a preoperative intraocular pressure between 40 and 49 mm Hg, two of the 12 patients (17%) with a preoperative intraocular pressure between 50 and 59 mm Hg, and one of four patients with an intraocular pressure more than 60 mm Hg developed a suprachoroidal hemorrhage. Because the preoperative intraocular pressure was highly correlated with the difference between the preoperative and the postoperative intraocular pressure, it was not possible to distinguish which factor was clinically more important.