Aim: To perform a correlation analysis between long-term intraocular pressure (IOP) variation parameters (mean, peak, and fluctuation) in patients with stable open-angle glaucoma (OAG). Materials and methods: A cross-sectional observational study was carried out, in which patients with stable OAG were consecutively enrolled. All patients had to have glaucomatous optic neuropathy and characteristic visual field (VF) defects. Key inclusion criteria were ≥5 VF tests, ≥3 disc photographs, and ≥3 years of follow-up without any changes in current medical regimen. Stable OAG was defined as nonprogressive VF results and absence of anatomical changes for at least 3 years. Long-term IOP variation parameters were obtained from isolated IOP measurements from each visit (minimum of five IOP measurements). The main outcome measure was the correlation between these IOP variation parameters. Results: Of the 63 patients studied, 37 (59%) were women, and the mean age was 61 ± 12 years. Among all the analyses, IOP mean and peak had the strongest correlation (r = 0.94; 95% CI = 0.92–0.97; p < 0.001). There were also significant correlations between IOP peak and IOP fluctuation (r = 0.84; 95% CI = 0.75–0.90; p < 0.001), and mean IOP and IOP fluctuation (r = 0.62; 95% CI = 0.43–0.75; p < 0.001). Conclusion: Most long-term IOP variation parameters evaluated seem to be highly correlated. Notably, the correlation between mean IOP and IOP peak was the strongest one. We believe this fact should be taken into consideration as their inclusion as individual variables in a multiple regression model could lead to misinterpretation of the data. Clinical significance: Different well-designed studies are conflicting regarding which long-term IOP variation parameter is more clinically relevant. Our findings suggest that collinearity issues could explain in part the discrepant results among these studies evaluating the relationship between long-term IOP variation parameters and glaucoma prognosis.