Purpose To examine the clinical features of undiagnosed open-angle glaucoma (OAG) in people who have attended an eye care provider within the previous 12 months and to suggest strategies to assist in the early detection of glaucoma. Design Population based cross-sectional study. Participants Permanent residents aged 40 years and older at recruitment during 1992 through 1996. Methods A cluster-stratified random sample of 4744 participants from the urban and rural cohorts was studied. Structured standardized interviews and dilated ocular examinations were conducted in all eligible participants. Data on demographic characteristics, prior knowledge of eye disease, use of eye care services, intraocular pressures, cup-to-disc ratios, visual fields, and photography of optic discs were obtained. All suspected glaucoma cases were submitted to a panel of 6 ophthalmologists to determine glaucoma diagnosis. Main outcome measures Clinical features of participants seen by eye health professionals within the previous 12 months who have previously undiagnosed OAG, previously diagnosed OAG, and no glaucoma. Results Thirty-five previously undiagnosed and 43 previously diagnosed participants had visited an optometrist or ophthalmologist or both in the previous 12 months. Age and gender were not significantly different between the undiagnosed and diagnosed glaucoma cases. After logistic regression, the type of eye professional seen (odds ratio [OR], 45.17; 95% confidence interval [95% CI], 5.89-346.17; P = 0.0002) and the presence of visual field defects (OR, 0.06; 95% CI, 0.01-0.69, P = 0.020) were the only statistically significant variables between the diagnosed and undiagnosed glaucoma groups. Conclusions Raised intraocular pressure should not be relied on as the only triggering factor in glaucoma investigations.