In 288 eyes of 114 low-tension glaucoma (LTG) patients, the mean, peak, trough and magnitude of the diurnal fluctuation of the intraocular pressure (IOP) determined over a period of 24 hours (diurnal IOP) were correlated with the IOPs recorded at the daytime office (office IOP), refraction, extent of visual field loss, age, sex, blood pressure and obesity index by the method of multivariate analysis. Only the mean office IOP was found to have a statistically significant contribution for estimating the mean, peak and trough of the diurnal IOP with the proportion (R) of 0.71 0.67 and 0.68 respectively. Furthermore, an analysis of the IOP data obtained from 118 LTG suspects using the receiver operating characteristics (ROC) curve showed that a patient whose peak diurnal IOP exceeds 21 mmHg could be detected with a sensitivity of 100% and a specificity of 46% if around-the-clock IOP measurements were carried out in patients whose mean office IOP are above 16 mmHg at least in one eye. In view of high prevalence of LTG in Japanese, estimation of mean diurnal IOP from mean office IOP and exclusion of primary open angle glaucoma using the mean office IOP of 16 mmHg as a cutoff IOP level are thought to be clinically useful.