The Heidelberg Retina Tomograph (HRT) was recently developed to measure two- and three-dimensional parameters of the optic disc. The instrument gives measurement results on an absolute scale; the validity of the scaling has never been proven in the living eye. Eight phakic eyes (eight patients) scheduled for subsequent pars plana vitrectomy were selected for this study. The optic disc was scanned using the HRT. During vitrectomy, the disc was measured directly using a modified retina spatula that had a 2.5-mm scale with a grading of 1/10 mm at the tip. The measurement was recorded with a high-resolution video system adapted to a Zeiss operation microscope. We used a vitrectomy contact lens with a flat surface. First we looked for reference points along the disc margin that could be clearly identified on both images, then we determined the distances between these points. Calculations from HRT data followed according to the software used in this instrument. Calculations from video images were related to the grading of the intraocular scale instrument. As determined using HRT software version 1.08, the mean ratio of HRT distances obtained by direct video measurements was 1.031 (SD, +/- 0.099; SE, +/- 0.035). No significant correlation between the ratio and the ametropia of the eyes could be verified (r = 0.506, P = 0.20). On average, the two-dimensional measurements of the HRT were correct. Furthermore, we used software version 1.07, which does not correct the image size depending on refraction, and found that the mean ratio of HRT distances obtained direct video measurements was 0.985 (SD, +/- 0.108; SE, +/- 0.038).(ABSTRACT TRUNCATED AT 250 WORDS)