Clinical Relevance Five days of at-home whitening with 10% carbamide peroxide (eight hours/day) produces a similar tooth whitening result as one in-office treatment with 25% hydrogen peroxide (one hour). SUMMARY Objective: This study evaluated the time necessary for at-home whitening (HW) to match the results of an in-office (OW) treatment, side effects and patients' preferences/perceptions. Methods: The tooth color change of 20 subjects was measured using a shade guide (BSG) and spectrophotometer (ES). Color difference was calculated: ΔE*= [(ΔL*) 2 + (Δa*ab) 2 + (Δb*ab) 2 ] 1/2 . The whitening treatments were randomly applied to the right or left maxillary anterior teeth, in-office, with 25% hydrogen peroxide or at-home, overnight, with 10% carbamide peroxide. The tooth color was evaluated at baseline, one day after OW, six days (five days after HW) and at 20 days (14 days after HW and 19 days OW). Subjects rated their tooth and soft tissue sensitivity (1–10 scale). The results were analyzed by two-way RM ANOVA/Tukey's and Mann-Whitney ( p <0.05). Results: At six days, the teeth that were treated with HW and OW presented ΔE* = 5.2 and 6.6, respectively, ΔBSG=3, and at 20 days, they presented ΔE* = 6.2 and 6.6, respectively, ΔBSG = 3. Less than 40% of the subjects experienced tooth sensitivity after OW and HW. No subjects experienced tooth and gingival sensitivity at 20 days. Seventy-four percent preferred HW over OW, 63% recommended OW and 100% recommended HW. Conclusion: While there was a subtle difference in ΔE* between HW and OW at six days, the measurement of ΔE* and ΔBSG agreed that five days of home whitening produced the same results as a single in-office treatment. The tissue and teeth sensitivity were mild and transient. Subjects preferred and would recommend HW over OW.