The objective of the present study was to assess the clinical efficacy of a dental bleaching system based on hydrogen peroxide with or without light activation. This randomized controlled trial evaluated the effect of the light when applied to the hydrogen peroxide by using a split-mouth design with 21 patients, with light activation in one hemi-arch but not in the other. The bleaching agent was QuickWhite 35% hydrogen peroxide and activation was conducted with a diode lamp (Luma Cool). The Classic Vita Guide was used to score tooth shades. Two consecutive applications of hydrogen peroxide were made to one hemi-arch, each light-activated for 10 min. The other hemi-arch was then identically treated but without light activation. After removal of the bleaching agent, the shade was re-scored and the Wilcoxon signed ranks test was used to compare differences in tooth shade values. The bleaching treatment produced significant shade changes (P < 0.01) in both hemi-arches. After treatment, there were no statistically significant differences between light-treated and non-light-treated tooth types (central incisors, lateral incisors, and canines). However, taking central incisor, lateral incisor, and canine as a group, comparison between each hemi-arch showed a significant effect in the hemi-arch with light activation (P < 0.05). The use of diode light with a 35% hydrogen peroxide gel slightly improved the dental bleaching.