The macular blood flow response to an induced change in intraocular pressure (autoregulation) was studied using the blue field entopic phenomenon in 11 open angle glaucoma patients, eight glaucoma suspects and 13 normal volunteers. A suction cup was used to raise the intraocular pressure (IOP) above its resting state (IOPrest). IOPmax, the highest acutely increased IOP for which blood flow can be maintained constant by autoregulation, was 24.9 +/- 1.5 mmHg (+/- 1 SD) in the glaucoma patients, 30.8 +/- 4.6 mmHg in the glaucoma suspects and 29.9 +/- 3.6 mmHg in the normal subjects. The values for IOPmax - IOPrest were 3.7 +/- 4.3 mmHg, 4.7 +/- 3.3 mmHg, and 14.3 +/- 3.1 mmHg, respectively. After the release of the suction cup, a hyperemic response was observed by 16 of 17 normal eyes, 10 of 14 glaucoma suspect eyes and only 9 of 19 glaucomatous eyes. These results suggest an abnormal autoregulation of macular retinal blood flow in open-angle glaucoma.