The aim of the study is to assess the correlation between ocular perfusion pressure (OPP) fluctuation and the severity of glaucoma. Forty-three patients with primary open-angle glaucoma (POAG) were enrolled in the study. All subjects had intraocular pressure (IOP) and blood pressure measured at three time points (7:00 a.m., 1:00 p.m. and 7:00 p.m.). OPP was calculated based upon blood pressure and IOP, and OPP fluctuation was calculated as the standard deviation of the three measurements. OPP fluctuation was correlated with a structural measure (cup-to-disk ratio) and two functional measures [mean deviation (MD) and pattern standard deviation (PSD) values from automated perimetry] with the Pearson's correlation coefficient. OPP fluctuation showed statistically significant correlation with both MD (r = -0.35, P = 0.01) and PDS (r = 0.44, P = 0.00). No correlation was found between OPP fluctuation and cup-to-disk ratio (r = 0.10, P = 0.48). POAG patients with higher OPP fluctuation have greater severity of disease as measured by functional parameters.