How long should we follow up the patients with glaucomatous visual field defects to confirm the diagnosis of primary open-angle glaucoma (POAG)? What is the most important examination for the diagnosis of POAG? In order to answer these questions, 108 eyes of 60 cases were followed up for more than 4 years. All of these subjects presented both open angle and glaucomatous field defects. In the patients who did not present the increment of the intraocular pressure (IOP) within one month from the first visit, the relationship between the increment of IOP after one month and provocative tests, sex, age and refractions were evaluated. Of these, 47 eyes showed an increment of IOP within one month from the first visit. The other 61 eyes, suspected of low tension glaucoma (LTG), were followed up for the further course of IOP. Of these, 25 eyes showed an increment of IOP after one month to 6 years from the first visit, and the other 36 eyes did not for more than 4 years to 21 years. Follow-up for at least one year is necessary to confirm the diagnosis of POAG. In order to predict the increment of IOP in the subjects with normal IOP and glaucomatous field defects, the ratio Po/C after drinking water is the most sensitive examination.