Purpose: The purpose of this study was to compare the efficacy and safety outcomes of a currently marketed, peroxide-containing, tray-based, tooth-whitening system to a peroxide-containing, "trayless" tooth-whitening system. Methods: Fifty-seven subjects, 12 to 17 years of age, participated in this study and were divided into 2 balanced groups. Twelve subjects received custom trays with 10% carbamide peroxide gel that they were instructed to wear overnight. Forty-five subjects received 10% hydrogen peroxide polyethylene strips to wear for 30 minutes twice a day. Teeth were bleached for 2 weeks. Digital image analysis measured color in B, L, and A color spaces, where B indicated yellowness, L indicated lightness, and A indicated redness. Oral examinations and interviews were used to ascertain any adverse events that may have occurred during treatment. Results: Fifty-one patients completed this study. Both whitening systems yielded significant (P <.001) color improvement, as evidenced by decreased yellowness, increased lightness, and decreased redness. Groups did not differ significantly (P >.39) regarding color improvement for B, L, or A on either the maxillary or mandibular teeth. Twelve subjects (27%) in the polyethylene strip group reported adverse events compared to 5 subjects (42%) in the tray-delivered group. Minor and transient tooth sensitivity and oral irritation were the most common adverse events. Conclusions: Both the daytime strip and overnight tray groups significantly (P <.0001) whitened teeth; there were no significant differences between the 2 groups in any of the color parameters; both whitening systems were well tolerated, and most adverse events were mild in severity.