In order to assess the effect of increased intraocular pressure on the corneal endothelium, we classified 254 patients into four groups: Group 1, those with normal intraocular pressures; Group 2, those with untreated ocular hypertension; Group 3, those with treated ocular hypertension; and Group 4, those with primary open-angle glaucoma. One eye of each patient underwent specular microscopy and pachymetry. The eyes in the four groups did not differ significantly as to central corneal endothelial cell density or central corneal thickness. These measurements were not related to sex, race, or intraocular pressure (P greater than .12 in all cases). Increasing age was associated with a decrease in central corneal endothelial cell density (P = .0001), but was not associated with a change in central corneal thickness (P = .22). There was no significant relationship between the use of topical ocular hypotensive medications and central corneal endothelial cell density (P = .38) or central corneal thickness (P = .07) in patients with ocular hypertension or primary open-angle glaucoma. Neither uncomplicated peripheral iridectomy nor trabeculectomy produced significant changes when preoperative measurements were compared to measurements made 12 weeks postoperatively (P greater than .30 in all cases). Two eyes with flat anterior chambers following trabeculectomy had substantial decreases in central corneal endothelial cell density.