According to various studies, treatment adherence of patients with primary open-angle glaucoma ranges from 5% to 80% and significantly influences the effectiveness of the therapy. Some authors report higher levels of adherence in patients receiving preservative-free medications. Purpose — to study the influence of treatment adherence in patients receiving preservative-free and preserved medications on the progression of primary open-angle glaucoma (POAG). Material and methods. This prospective multicenter non-interventional study was conducted in clinical setting and included 504 patients with primary open-angle glaucoma receiving drug therapy; 51 ophthalmologist was involved in the management of patients’ diaries. Results. The percentage of patients who did not reach target IOP was three times lower in the group with high adherence, in comparison to patients with medium and low adherence (7% against 22%, respectively). Development of side effects was one of the factors influencing treatment adherence, their rates and expressiveness were much lower in the preservative-free group compared with the group receiving preserved medication (13.9% against 35.4%). The rate of patients who did not reach target IOP was two times lower in the group of preservative-free medication (10% against 21%). Conclusion. The study confirms that preservative-free glaucoma medication promotes higher adherence to treatment in clinical setting predominantly due to the reduction of quantity and severity of side effects leading to treatment gaps, which results in increased effectiveness and better stabilization of glaucoma.