OBJECTIVE To determine whether normal-tension glaucoma, defined as a condition in which glaucomatous optic nerve and visual field changes exist without documentation of intraocular pressure greater than 21 mm Hg or other apparent cause for these changes, is overdiagnosed in patients with decreased central corneal thickness and curvature. METHODS Twenty-one patients with normal-tension glaucoma were compared with 25 patients with primary open-angle glaucoma and 27 age-matched healthy subjects. Corneal thickness was determined by ultrasonic pachymetry. Corneal curvature was determined using a keratometer. Eyes with corneal pathology or previous intraocular surgery were excluded. RESULTS Mean corneal thickness +/- SD in 21 eyes of 21 patients with normal-tension glaucoma was 0.521 +/- 0.037 mm, significantly (P = .0028) lower than in 25 eyes of 25 patients with primary open-angle glaucoma (0.556 +/- 0.035 mm) and 27 eyes of 27 healthy subjects (0.555 +/- 0.034). Mean corneal curvature in the three groups was not appreciably different: 43.90 +/- 1.81 diopters, 43.66 +/- 1.68 diopters, and 44.36 +/- 1.13 diopters in the patients with normal-tension glaucoma and primary open-angle glaucoma and the healthy subjects, respectively. CONCLUSIONS Corneal thickness is significantly reduced in patients with normal-tension glaucoma compared with patients with primary open-angle glaucoma (P = .0028) and normal subjects (P = .0037). This may lead to underestimation of intraocular pressure and misdiagnosis in some of these patients. Corneal curvature was similar in patients with normal-tension glaucoma and primary open-angle glaucoma and in healthy subjects.