Objective: To compare clinical efficacy and safety between user-formed and professionally fabricated trays for Rembrandt Plus Tooth Bleaching Kit. Methods: Subjects (32) were randomly assigned to two groups (16/group). Impressions were obtained for fabricating custom trays. At the baseline, demographic information and medical history were collected from each subject, and clinical examinations included oral tissues,tooth shade of maxillary anterior teeth measured by Bioform Color Ordered Shade Guide (Dentsply International Inc., York, PA), and tactile tooth sensitivity. Subjects were either instructed to prepare the trays following instructions included in the kit or provided with the custom trays. Both groups used the same Rembrandt Plus gel for 60 minutes each time, twice daily, for 14 days, following the instructions. Bioform shade data were analyzed using the Student t-test, and oral tissue and sensitivity results were descriptive. Results: The average baseline Bioform shade values (user-formed: 13.97; professional: 12.61) were comparable between the two groups. The shade reductions after 14-day treatments were 10.11 and 10.13 for the user-formed tray and professional tray groups, respectively. The average shade at the 14-day (user-formed: 3.86; professional: 2.48) and the reductions were not significantly different between the two groups. One subject in each group reported mild tactile sensitivity at the baseline. At 14-day, severe sensitivity was found in one tooth of a different subject in the professional tray group, while three subjects in the user-formed tray group reported mild to moderate sensitivity. Mild gingival erythema was found in four subjects at 14-day, with one using professional tray and three using user-formed trays. No clinical evidence of other adverse effects on oral tissues was detected. Conclusion:Rembrandt Plus Tooth Bleaching Kit is effective in whitening teeth with either professionally fabricated or user-formed trays, and both types of tray offer similar whitening efficacy. Support by Den-Mat Corporation.