Objective: To evaluate the intraocular pressure (IOP)-lowering effects of adjunctive medications when added to 0.005% latanoprost taken once daily. Methods: Pertinent publications were identified through systematic searches of PubMed, Embase, and the Cochrane Controlled Trials Register. Randomized clinical trials with over 85% of patients presenting with primary open-angle glaucoma or ocular hypertension who were treated with the combination treatment of latanoprost were selected. The pooled additional IOP-lowering effects at 1–3 months after a run-in phase of at least 2 weeks on 0.005% latanoprost once daily were calculated using the random effects model. Results: Nine randomized clinical trials were included. The mean pooled IOP reductions were 3.3 mm Hg (95% CI: 2.1–4.5) at trough and 4.4 mm Hg (95% CI: 3.4–5.4) at peak when adding 0.5% timolol once daily, 2.6 mm Hg (95% CI: 1.9–3.3) at trough and 3.8 mm Hg (95% CI: 2.5–5.2) at peak when adding 0.1/0.15% brimonidine twice daily, 2.6 mm Hg (95% CI: 1.7–3.4) at trough and 3.1 mm Hg (95% CI: 2.6–3.6) at peak when adding 2% dorzolamide twice daily, 2.4 mm Hg (95% CI: 2.0 –2.8) at trough and 2.7 mm Hg (95% CI: 2.2–3.2) at peak when adding 0.5% timolol twice daily, and 2.8 mm Hg (95% CI: 1.5–4.1) at trough and 1.8 mm Hg (95% CI: 1.2–2.3) at peak when adding 1% brinzolamide twice daily. Conclusions: The addition of brimonidine, dorzolamide, timolol, or brinzolamide can further lower IOP in eyes being treated with latanoprost. Timolol 0.5% once daily might be the most effective adjunctive medication.