Objectives: The degree of effectiveness of tooth bleaching is influenced by many factors. The purpose of this study is to evaluate the differentiation of effectiveness related to the original tooth color. Methods: One of the most popular tooth bleaching systems (Opalescence 10% PF, Ultradent, USA) was applied for 32 subjects who had upper maxillary anterior dentitions without any restorations and bleaching history. Tooth bleaching was performed for all subjects with a minimum of 4 hours per day for 14 days. The tooth color of the central incisors (I) and canines (C) was measured by a multi-spectrophotometer (MSC-2000, Olympus, Japan) over three regions on the tooth (incisal, body and cervical) prior to bleaching and after completing the bleaching. CIELAB color coordinates of L*, a* and b* were calculated and color differences (dE) were obtained. Correlation between color difference dE and CIELAB color coordinates of the original tooth color prior to bleaching was analyzed by Spearman’s correlation coefficient test. Results: Correlation coefficients were shown:

	dL* (I)	da* (I)	db* (I)	dL* (C)	da* (C)	db* (C)
Incisal	-0.554	0.301	0.696	-0.72	0.286	0.724
Body	-0.61	0.418	0.720	-0.698	0.497	0.727
Cervical	-0.633	0.483	0.778	-0.654	0.426	0.696

Effectiveness of tooth bleaching tested in this study indicated mean dE values from 8.1 to 9.3 for incisors and from 9.1 to11.2 for canine. Correlation coefficients between dE and color coordinates of b* were from 0.696 to 0.778 that indicated a strong correlation, but there was no strong correlation between dE and a*. On the other hand, L* had a negative coefficients indicating moderate correlation. Conclusions: It was concluded that tooth bleaching in this study had a higher level of effectiveness for b* and L* values than a* values.