OBJECTIVE To evaluate the effect of phacoemulsification on intraocular pressure (IOP) control in eyes with a previous functioning filtering bleb and no glaucoma medication. METHODS Prospective, nonrandomized comparative (self-controlled) trial. METHODS Forty-seven patients (49 eyes) who underwent phacoemulsification after successful trabeculectomy, with at least 12 months of follow-up. METHODS Clear corneal phacoemulsification and implantation of a foldable intraocular lens in eyes that underwent a previous successful trabeculectomy. The time between both procedures was always greater than 1 year. METHODS Preoperative and postoperative IOP, the number of glaucoma medications, bleb appearance, and visual acuity were recorded at each follow-up examination. Success was defined as no need for glaucoma medications, bleb needling, or further glaucoma surgery for IOP control after phacoemulsification. Preoperative and intraoperative factors were evaluated for an association with postoperative failure using Kaplan-Meier survival analysis. RESULTS The mean (+/- standard deviation) IOP before phacoemulsification was 12.24 (+/- 4.68) mmHg, and it increased 3.94, 3.76, 1.39, 2.04, and 1.57 mmHg on the first postoperative day, after 1, 6, and 12 months, and at the last visit, respectively. At each interval, the mean IOP was significantly higher than the preoperative value (P = 0.000, 0.000, 0.049, 0.01, and 0.01, respectively). Nevertheless, the mean IOP after phacoemulsification was always lower than before trabeculectomy (P = 0.000). At the last visit, glaucoma medication was required in 17 eyes (34.7%). The success rates after phacoemulsification were 83.6%, 68.2%, and 55.7% at 6 months and 1 and 2 years, respectively (Kaplan-Meier survival analysis). The number of glaucoma medications used increased at all follow-up visits (P < 0.005). Bleb size decreased after phacoemulsification (P = 0.000). An IOP before phacoemulsification of greater than 10 mmHg was associated with postoperative failure (P = 0.002). Similarly, bleb failure and the need for glaucoma medication were associated with higher IOPs before phacoemulsification. CONCLUSIONS Phacoemulsification significantly increased IOP and the number of glaucoma medications in eyes with preexisting functioning filtering blebs. Eyes with higher IOPs before phacoemulsification had worsened postoperative IOP control and bleb failure.