29 patients with initial primary open angle glaucoma were examined using Heidelberg retinal tomograph III (HRT III), factors influencing measurements reproducibility and variability were studied. The following factors were found to cause outliers in stereometric parameters of the optic nerve head and retinal nerve fiber layer: standard reference height showing large difference of measurements or very high values, significant difference of scan depth, small optic disc, astigmatism, old age, interobserver variability of disc area (contour line position). Considering these factors one can determine indications for control examination thus improving method accuracy.