Phacoemulsification has been linked to lowered intraocular pressure (IOP) in patients with glaucoma, ocular hypertension, anatomic narrow angles, and in glaucoma suspects, but the magnitude of change has varied. Retrospective cohort study. Patients with glaucoma treated from June 2010 through May 2015 who underwent phacoemulsification (surgical group) were matched to patients who did not (nonsurgical group) for age, gender, type of glaucoma, baseline IOP, and number and type of glaucoma medications. Electronic medical record information was used to compare the matched surgical and nonsurgical groups. Change in IOP, change in number of glaucoma medications, and likelihood of a glaucoma procedure within 36 months after phacoemulsification. Intraocular pressure measures were obtained from Goldmann applanation tonometry when available (45%), and otherwise with the iCare tonometer (iCare USA, Raleigh, NC), the Tono-Pen (Reichert Technologies, Depew, NY), noncontact tonometry, and pneumotonometry. Among 16 169 matched pairs, average IOP after the index date was lower in the surgical than nonsurgical group throughout follow-up to 36 months. The difference was greatest during months 1 through 18, during which IOP increased by 0.22 mmHg from 16.49 mmHg in the average nonsurgical patient and decreased by 0.99 mmHg from 16.50 mmHg in the average surgical patient (difference in difference, 1.21 mmHg; 95% confidence interval [CI], 1.12-1.30 mmHg). The difference in difference was greatest for patients with ocular hypertension (2.00 mmHg) and for patients with preoperative IOP of 20 mmHg or more (2.46 mmHg). By 30 to 36 months, 5% (95% CI, 4%-6%) fewer surgical patients used an ophthalmic medication. In the surgical group, the odds of selective laser trabeculoplasty were reduced in patients with ocular hypertension (odds ratio [OR], 0.27; 95% CI, 0.10-0.74) or glaucoma suspects (OR, 0.31; 95% CI, 0.20-0.47), whereas the odds of glaucoma surgery were elevated in surgical patients with primary open-angle glaucoma (OR, 1.48; 95% CI, 1.08-2.01). The association of phacoemulsification for cataract with IOP reduction was lower than in past referral-based studies. Surgeons should expect to reduce IOP approximately 1 to 2 mmHg with phacoemulsification in patients with preoperative IOP of less than 20 mmHg.