The aim was to investigate the effectiveness of the fixed drug combination dorzolamide 2%/timolol 0.5% as monotherapy, substitutive and as adjunctive therapy in patients with primary open angle glaucoma. 130 patients were divided in three different study groups and treated by the combination dorzolamide 2%/timolol 0.5% as monotherapy, substitutive therapy and additional therapy. After three months using fixed combination dorzolamide 2%/timolol 0.5%, the mean IOP decreased by 5.6 mmHg. After prescribing substitutive therapy with fixed drug combination dorzolamide 2%/timolol 0.5% for 3 months, the intraocular preassure increased by 0.13 mmHg. Three months after the treatment with fixed combination dorzolamide 2%/timolol 0.5% as an additional antiglaucomatous drug, we have noticed a decrease of the mean IOP of 3.28 mmHg. Itching and soarness were reported by 43% patients. In conclusion, glaucoma drug therapy with fixed drug combination dorzolamide 2%/timolol 0.5% achieves effective lowering of intraocular preassure levels as monotherapy or as adjunctive therapy. As a substitutive therapy, the combination had the same IOP lowering effect. Simple drug administration and good drug tolerance indicate the effectiveness of the same drug.