Introduction: The purpose of this study was to evaluate the effect of a highly filled light-cured sealant (HFLCS) on the bond failure rate of metal brackets bonded with a conventional composite adhesive in vivo for 18 months. Methods: In 30 consecutively treated patients, the teeth were divided into 2 groups based on the universal numbering system (1-32). Metal brackets were bonded to all odd-numbered teeth (n = 272) with a conventional composite resin; they served as the controls. Metal brackets were also bonded to all even-numbered teeth with the same composite resin but with a different sealant: HFLCS (Pro Seal, Reliance Orthodontic Products, Itasca, Ill). A total of 544 teeth were bonded and followed for 18 months. The difference between the failure rates (expressed as percentages) was compared with the McNemar test. Results: In the control teeth, the bond failure rate was 3.7%, and the mean failure time was 149 days. The teeth pretreated with Pro Seal had a bond failure rate of 2.9% and a mean bracket failure time of 127 days. Comparison of the bond failure rates did not show statistical significance (P = 0.81). Conclusions: HFLCS did not adversely affect the failure rate and time of metal brackets when it was used instead of conventional sealant in a composite resin bonding system. © 2009 American Association of Orthodontists.