A significant amount of variability exists between observers in designating cup/diac (C/D) ratios. Further, different methods are used for evaluating the size of the cup. On method uses a combination of contour and color, the other specifically measures contour and pallor separately. This study confirms that these methods yeild different numerical results. Interestingly, the "cupping/pallor" observers show no significant difference among one another in C/D determination which may support the notion that cupping observations are more consistent than "standard" C/D ratios. Though both groups feel the cup is larger on stereo compared to monocular viewing, the "cupping/pallor" group demonstrated much greater differences in this regard. However, the evaluation of pallor by the latter group corresponded closely to the C/D ratios of the "standard" group both for nonstereo and stereo. An individual observer is reasonably consistent on repeat evaluation, but at times, inconsistency is quite substantial. These inconsistencies lead to the conclusion that C/D ratios are an inexact method of recording the status of a disc. Except for considerable changes over time, this numerical method is probably not reliable in checking for small disc changes. However, the disc alone can provide clues as to whether it is physiologic or pathologic. Nevertheless, even with expert observers, significant variability exists in interpretation of an optic disc in this regard. Individuals who evaluate the disc cup by cupping and pallor tend to call discs more pathologic than other observers. The clinical implications of this study suggest that certain observers are more accurate than others and certain discs are more easily evaluated than others. Yet, no one method seems foolproof and no specific criteria can as yet distinguish a normal from an abnormal disc. Stereo color transparencies are evaluated more accurately than are nonstereo color prints. Vessel detail and pallor patterns are probably the most useful disc characteristics to observe. In this regard, a C/D ratio does nothing to indicate whether a disc is normal or not. A statement as to the observer's opinion should be made for each disc evaluation in addition to recording the appearance of the disc.