The sizes for the perceptible or acceptable color difference measured with instruments vary by factors such as instrument, material, and color-difference formula. To compensate for disagreement of the CIELAB color difference (Delta E-ab*) with the human observer, the CIEDE2000 formula was developed. However, since this formula has no uniform color space (UCS), DIN99 UCS may be an alternative UCS at present. The purpose of this study was to determine the correlation between the CIELAB UCS and DIN99 UCS using dental resin composites. Changes and correlations in color coordinates (CIE L *, a *, and b * versus L-99, a(99), and b(99) from DIN99) and color differences (Delta E-ab* and Delta E-99) of dental resin composites after polymerization and thermocycling were determined. After transformation into DIN99 formula, the a value (red-green parameter) shifted to higher values, (aid the span of distribution was maintained after transformation. However, the span of distribution of b values (yellow-blue parameter) was reduced. Although color differences with the two formulas were correlated after polymerization and thermocycling (r = 0.77 and 0.68, respective), the color coordinates and color differences with DIN99 were significantly different from those with CIELAB. New UCS (DIN99) was different,from the present CIELAB UCS with respect to color coordinates (a and b) and color difference. Adaptation of a more observer-response relevant uniform color space should be considered after visual confirmation with dental esthetic materials. (C) 2006 Wiley Periodicals. Inc.