This study examined the changes in retinal blood flow and vessel diameter after IOP reduction in high- and low-pressure glaucoma; exfoliation glaucoma (ExG) and normal-tension glaucoma (NTG). Glaucoma progression is seen at times in eyes despite IOP reduction and changes in blood flow have been previously detected in the ocular vasculature. The study recruited consecutive patients attending for deep sclerectomy; 17 eyes with ExG and 20 with NTG. Blood flow in the temporal peripapillary retina was measured with scanning laser Doppler flowmetry, and retinal vessel diameters were evaluated using the retinal vessel analyser. Investigations were carried out just before and three months after the operation and presented to include systemic flow, diastolic flow, mean flow and pulsation index in the temporal peripapillary retina. The results indicated that there were no differences between age, visual fields and number of topical medication. Preoperative IOP was significantly higher in ExG than in NTG (median 26 mmHg). Surgery reduced IOP significantly both in ExG eyes. After the operation, systolic retinal flow was significantly reduced in ExG eyes, whereas in NTG, HRF parameters remained unchanged. Preoperatively, the central retinal artery equivalent and arteriovenous ratio were higher in ExG than in NTG eyes. After IOP reduction, both were reduced in ExG eyes, but remained unchanged in NTG. The study concluded that there are local changes in blood flow related to IOP. Prior to IOP reduction, arterial diameter was larger in ExG eyes than in NTG eyes and this resulted in vasoconstriction and reduction of flow in ExG, whereas in NTG, both vessel diameter and retinal flow remained unchanged.