OBJECTIVES: To determine the effect of central corneal thickness (CCT) on the measurement of intraocular pressure (IOP) and on the resultant reclassification of patients as having primary open-angle glaucoma (POAG), normal tension glaucoma (NTG), or ocular hypertension (OHT). METHODS: Intraocular pressure (Goldmann applanation tonomety) and CCT (ultrasound pachymetry) were measured in 22 patients with NTG, 49 with POAG, 44 with OHT and in 18 control subjects. The CCT was used to obtain a corrected value for the IOP and to reclassify the type of glaucoma. RESULTS: There was no significant difference in CCT between controls (552 +/- 35 microns) and patients with POAG (543 +/- 35 microns), but the CCT in the group with NTG (521 +/- 31 microns) was significantly lower than that in the control group or the group with POAG (P < .001), and the CCT in the group with OHT (583 +/- 34 microns) was significantly higher than in controls or patients with POAG (P < .001). Correcting IOP for corneal thickness, 31% of the patients with NTG could be reclassified as having POAG, and 56% of the patients with OHT as normal. CONCLUSIONS: Patients with NTG have a thinner CCT than do patients with POAG or controls. Underestimation of the IOP in patients with POAG who have thin corneas may lead to a misdiagnosis of NTG, while overestimation of the IOP in normal subjects who have thick corneas may lead to a misdiagnosis of OHT.