The aim of this study was to determine the intra- and inter-observer variation in the use of a system designed for exact measurements from standard optic nerve head photographs. The commercially available system consisted of a colour CCD Videocamera, a dedicated frame grabber and customized software run on a IBM AT compatible computer. Masked measurements were made 3 times by 2 observers, from stereophotographs of the optic nerve head of 56 eyes from 30 glaucoma suspects. The cup was defined on the basis of contour, not pallor and the disc area was defined as the area inside Elschnig's ring. Intra-observer variances were 0.001 +/- 0.001 mm2 for cup area (mean +/- SD), 0.002 +/- 0.002 mm2 for disc area and 0.002 +/- 0.003 mm2 for rim area. These values for intra-observer variance were comparable with the results obtained using manual planimetric techniques. Intra-observer variance for disc area was significantly larger for the less trained of the two experienced observers. Inter-observer variances were 0.004 +/- 0.009 mm2 for cup area, 0.008 +/- 0.013 mm2 for disc area and 0.009 +/- 0.014 mm2 for rim area. These inter-observer variances were significantly larger than those previously reported for manual planimetry. The absolute differences between the two observers ranged from -0.35 to +0.20 mm2 (-0.08 +/- 0.11 mm2) for cup area, from -0.38 to +0.15 mm2 (-0.08 +/- 0.11 mm2) for disc area and from -0.29 to +0.34 mm2 (-0.06 +/- 0.12 mm2) for rim area.(ABSTRACT TRUNCATED AT 250 WORDS)