We evaluated changes in intraocular pressure in eyes with primary open-angle glaucoma after inversion into a totally dependent position with the head down and compared them with the changes intraocular pressure in healthy nonglaucomatous eyes. Five minutes after inversion, intraocular pressure increased from 16.8 mm Hg +/- 2.8 (sitting) to 32.9 mm Hg +/- 7.9 in normal eyes and from 21.3 mm Hg +/- 2.3 (sitting) to 37.6 mm Hg +/- 5.0 in glaucomatous eyes. Although visual fields were unchanged after inversion, glaucomatous eyes may be particularly susceptible to such changes and unable to withstand the transient but significant increase in intraocular pressure that occurs after assuming an inverted position. We recommend that patients with ocular hypertension or glaucoma refrain from this activity.