PURPOSE: To test the hypothesis that the correlation between office-hour intraocular pressure (IOP) and peak nocturnal IOP is weakened after using a prostaglandin analog. DESIGN: Before-and-after study. METHODS: Twenty-four-hour IOP data obtained in a sleep laboratory of 51 patients (22 patients with open-angle glaucoma and 29 patients with ocular hypertension) were reviewed. Patients had no IOP-lowering medication upon study entry and were then treated with prostaglandin monotherapy for 4 weeks. Measurements of IOP were taken every 2 hours in the sitting and supine positions during the diurnal/wake period (7:30 AM-9:30 PM) and in the supine position during the nocturnal/sleep period (11:30 PM-5:30 AM). Individual and average IOP readings during office hours (9:30 AM-3:30 PM) and peak IOP during the nocturnal/sleep hours were analyzed using the Pearson correlation coefficient and linear regression. RESULTS: There were statistically significant correlations for all the paired variables for the analyses. Average office-hour IOP had a higher correlation with peak nocturnal IOP than individual office-hour IOP. After the treatment with prostaglandin analog, the correlation between average office-hour IOP and nocturnal peak IOP in the sitting position (r = 0.373) and the supine position (r = 0.386) were reduced from the sitting baseline (r = 0.517) and the supine baseline (r = 0.573) in right eyes. Similar change patterns appeared in left eyes. CONCLUSION: There is a correlation between office-hour IOP reading and peak nocturnal IOP under no IOP-lowering treatment as well as under prostaglandin monotherapy. The strength of correlation was weaker under the treatment compared with baseline. (C) 2020 Elsevier Inc. All rights reserved.