Our objective was to study the intraocular pressure (IOP) in open-angle glaucoma or ocular hypertensive patients during long-term treatment with latanoprost. A total of 532 patients treated with 0.005% latanoprost were enrolled, including 493 and 113 patients treated for 6 and 24 months, respectively. Mean IOP was analyzed with the analysis of variance technique. The risk of treatment failure was analyzed with survival analysis technique. After 2 weeks of latanoprost treatment, the mean IOP was reduced 8.2 (32%) and 8.9 (34%) mm Hg in the subgroups of patients treated for 6 and 24 months, respectively. The change in mean IOP during 2 years of latanoprost treatment was not statistically significant (p = 0.15). Patients with primary open-angle glaucoma or ocular hypertension showed an 86% and 97% chance of receiving a sufficient IOP reduction with latanoprost (p < 0.01), repectively. The initial mean IOP reduction was maintained throughout the 2 years of treatment.