It is generally believed that some degree of defective color vision is frequently acquired along with certain ocular and systemic disorders. Precise definition of the nature and extent of the color defects has not been possible because of the limitations inherent in the tests currently available for clinical use. We undertook to define the defects in terms of thresholds of discrimination to each color, plotted on a color circle similar to Munsell's uniform chromaticity scale diagram. Furthermore, we were able to construct a symmetrical-type of colorimeter which plots thresholds directly on a printed circle without having to read and interpret scales or dials. Good correlation of subtle defects with certain disorders was confirmed, and may eventually be helpful in diagnosing and following some types. This system of threshold measurement was found to provide more information about color vision than any one of the conventional tests, or all of them combined.