Background. Tooth sensitivity has been reported with in-office tooth bleaching. The authors conducted a study to evaluate whether the use of a desensitizing agent before in-office bleaching decreased this sensitivity. Methods. Before in-office bleaching with a 35 percent hydrogen peroxide gel (three applications of 15 minutes each), clinicians applied a placebo gel on the buccal surfaces of all teeth in 15 participants in the control group, which was left undisturbed for 10 minutes. The clinicians applied a desensitizing agent to the teeth of 15 participants in the experimental group in the same manner as that described for the control (placebo) group. They repeated this protocol one week later. Patients recorded their tooth sensitivity on a 0-to-4 scale. The authors used t test (α =.05) to compare the tooth color changes and the intensity of tooth sensitivity between groups at baseline and immediately after the first and second appointments. The authors used Fisher exact test and t test (α =.05), respectively, to analyze the percentage of patients with tooth sensitivity and its intensity. Results. The use of a desensitizing gel did not affect the bleaching efficacy. Seven participants (46.7 percent) in the experimental group and 13 participants (86.7 percent) in the control group experienced tooth sensitivity (P <.05). The intensity of sensitivity was statistically significantly higher for participants in the control group (P <.05). Conclusions. The use of a desensitizing gel (5 percent nitrate potassium/2 percent sodium fluoride) before in-office bleaching did not affect the bleaching efficacy but did reduce tooth sensitivity. Clinical Implications. The use of a desensitizing gel before in-office tooth bleaching can reduce tooth sensitivity, the most common side effect associated with this procedure.