Measurable structural alterations of the optic nerve head may precede visual field abnormalities in early open-angle glaucoma. The authors studied the optic nerve heads of 10 patients with unilateral visual field loss from primary open-angle glaucoma, and 12 age- and sex-matched normal subjects. Topographic optic nerve head parameters were measured with a system of computerized image analysis (Rodenstock Analyzer, G. Rodenstock Instrumente GMBH, Munich, W. Germany). In patients with asymmetric primary open-angle glaucoma, eyes with normal visual fields had a slightly larger mean (+/- standard error of the mean) disc rim area (0.90 +/- 0.04 mm2) than eyes with glaucomatous visual field defects (0.78 +/- 0.05 mm2). However, both sets of eyes in the asymmetric primary open-angle glaucoma patients had smaller mean disc rim areas (P less than 0.0007) than did the control group (1.27 +/- 0.09 mm2). These findings support the hypothesis that loss of the optic disc rim can be detected before perimetric abnormalities develop in primary open-angle glaucoma.