The authors retrospectively studied intraocular pressure (IOP) changes over time in normal-tension glaucoma (NTG). The materials consisted of 160 eyes of 80 patients who fulfilled the diagnostic criteria for NTG including a continuous daily IOP profile of less than 21 mmHg. They were followed for 12 to 64 months (mean: 31 months) at 1 to 3 month intervals. Eleven eyes (6.9%) in 9 patients (11.3%) showed IOP equal to or greater than 21 mmHg at least once during the follow-up period. The probability of IOP remaining below 21 mmHg for 5 years was 89.1% for each eye and 80.6% for each case according to the Kaplan-Meier life-table method. A discriminant analysis with stepwise variable selection revealed that higher maximum IOP in daily profile, positive history of diabetes mellitus, a family history of glaucoma, male sex, positive history of disc hemorrhage, low diastolic blood pressure, earlier visual field change and concordance in IOP and visual field were significant prognostic factors for subsequent IOP increase.