Objective: A double-blind study to evaluate clinical tooth whitening efficacy and safety of trèswhite by Opalescence (Ultradent, South Jordan, UT) compared to a placebo.  Methods: Sixty healthy adults were recruited with IRB approval and informed consent.  Subjects were randomly assigned to two groups (30/group), balanced with baseline Vitapan Classical shade.  The subjects were instructed to use the assigned material once for 60 minutes daily for 10 days.  Clinical evaluations at the baseline and post-treatment included tooth shade, tooth sensitivity (tactile and thermal), gingival index (GI), and oral tissues.  The age was analyzed using the two-sample t-test, while the shade, tooth sensitivity and GI data were examined using the Mann-Whitney U-test at the significance level of α=0.05.  Findings of the oral tissue examination were descriptive.  Results: Fifty-five subjects, 27 in trèswhite group and 28 in placebo group, completed the study.  The demographical data and the average baseline shade were comparable between the two groups.  The shade reduction after 10-day treatment with trèswhite was 6.12, which was significantly greater (p<0.0001) than that of the placebo (0.72).  The within-effect of the trèswhite group was also significant (p<0.0001) while it was non-significant for the placebo group (p=0.114).  The effect of treatment on the incidence of tooth sensitivity, which was mild to moderate, was low (+0.05 and +0.01 for trèswhite and placebo, respectively).  The average GI after the treatment decreased for the trèswhite group (-0.02) but increased in the placebo group (+0.03).  Data analysis showed no significant differences in within- and between-effects in tooth sensitivity and GI results.  No oral tissue abnormalities associated with the use of trèswhite and placebo were detected.  Conclusion: The use of trèswhite by Opalescence once for 60 minutes daily for 10 days is effective and does not cause significant tooth sensitivity or gingival irritation.