OBJECTIVE To compare the effects of cataract surgery on intraocular pressure (IOP) according to preoperative factor in primary open-angle glaucoma (POAG) and primary angle-closure glaucoma (PACG). METHODS The medical records of 75 POAG and 95 PACG patients who underwent cataract surgery were reviewed. We classified POAG patients with a preoperative peak IOP of less than 31 mmHg and less than three medications used before surgery and PACG patients with a peak IOP of less than 42 mmHg, less than three medications used, and peripheral anterior synechiae of less than four clock hours into group 1. Patients with levels exceeding these thresholds were classified into group 2. The IOP, numbers of medications, and success rates were compared between two groups. RESULTS At 36 months after surgery, IOP reduction in group 1 was significantly greater than that in group 2 among POAG patients (-1.7 ± 2.1 vs. -0.6 ± 2.0 mmHg, p = 0.021); however, there was no significant difference between the two groups for PACG patients (-2.5 ± 2.0 vs. -2.2 ± 3.3 mmHg, p = 0.755). The medication changes were similar between the two groups for both POAG and PACG patients. The success rate at 36 months was significantly higher in group 1 than in group 2 for POAG patients (66.7% vs. 35.7%, p = 0.009), but there was no significant difference between the two groups for PACG patients (79.1% vs. 69.2%, p = 0.264). CONCLUSIONS For patients with relatively low peak IOP who used fewer medications before surgery, cataract surgery alone was effective for IOP control in both POAG and PACG patients. Conversely, For POAG patients with a history of higher peak IOP and who used more medications, cataract surgery was not effective in lowering IOP, whereas it resulted in relatively good IOP values in PACG patients.