PURPOSE: To investigate the dependence upon intraocular pressure (IOP) of the progression of visual field defects in eyes with primary open-angle glaucoma (POAG), in which the mean IOP was maintained at < or =21 mm Hg. METHODS: This study involved 100 eyes with POAG, which were followed up for > or =5 years. The mean IOP levels were maintained at < or =21 mm Hg during the follow-up period. The relationship between the IOP and the progression of visual field defects, which was scored using the Advanced Glaucoma Intervention Study criteria, was investigated retrospectively. RESULTS: Compared with the baseline scores, the visual field defect scores had significantly worsened by the end of the follow-up period (P<0.0001, Wilcoxon paired signed rank test). The change in the visual field defect score (2.5+/-0.5) in eyes with average IOP levels of > or =16 mm Hg (n=36) was significantly greater (P=0.031, Mann-Whitney U test) than the change (1.3+/-0.3) in eyes with average IOP levels of <16 mm Hg (n=64). Moreover, IOP of > or =18 mm Hg made a major contribution to the aggravation of visual field defects in eyes with POAG. CONCLUSIONS: Eyes with POAG and with mean IOP levels maintained at < or =21 mm Hg underwent IOP-dependent progression of their visual field defects. Our results suggest that further IOP lowering would be beneficial in such cases.