Bleaching materials containing 30% hydrogen peroxide have been used successfully for the treatment of discolored non-vital teeth. Intracoronal application of these materials was occasionally associated with the development of external root resorption. Extracted human teeth with intact crowns were discolored in vitro and bleached with three preparations of sodium perborate. These preparations included: sodium perborate with 30% hydrogen peroxide, sodium perborate with 3% hydrogen peroxide and sodium perborate with water. The bleaching materials were placed in the pulp chamber of the discolored teeth and sealed with IRM for 14 days. They were replaced with fresh preparations after 3 and 7 days. The coronal tooth shades were evaluated after 3, 7 and 14 days and a comparison of the bleaching success of the groups was made at each interval. It was found that after 14 days and three bleachings there was no significant difference in success between the groups. It is therefore recommended that sodium perborate be used in combination with water rather than with hydrogen peroxide to reduce the risk of post-bleaching external root resorption.