Intraocular pressure (IOP)-dependent retinal vascular changes were investigated in 33 eyes of 33 adult chronic open-angle glaucoma (COAG) patients by measuring major retinal vascular calibers at the optic disc border before and after mean (+/- standard deviation) IOP reduction from 35.3 (+/- 7.2) to 16.5 (+/- 4.7) mmHg for 11.2 (+/- 13.5) weeks. Both mean arterial and venous calibers were significantly reduced (P less than 0.0001 for each), from 87.8 (+/- 14.2) and 128.3 (+/- 20.8) microns at high IOP to 82.0 (+/- 13.8) and 121.4 (+/- 18.5) microns at low IOP. Arterial and venous caliber decreases correlated positively with magnitude of IOP reduction (r = 0.503, P less than 0.01 and r = 0.555, P less than 0.001, respectively). While the IOP-dependent retinal arterial caliber change was highly significant in patients 55 years of age or younger (r = 0.636, P less than 0.01) and in the overall study group, it was not significant in patients older than 55 years (r = 0.205, P greater than 0.1). Age seems to be more influential than magnitude of IOP reduction in patients older than 55. Thus, diminished IOP-dependent retinal arterial caliber change in the elderly may be one factor contributing to the higher incidence and prevalence of glaucoma in this population.