Hydrogen peroxide has been advocated for many years as an oral rinse useful in control of various oral conditions. Several authorities, however, have suggested that this material may be harmful to oral tissues, especially if the tissues have been previously injured. This article presents two case reports demonstrating harmful oral effects from hydrogen peroxide rinses. The findings suggest that oral hygiene techniques emphasizing the use of 3% hydrogen peroxide in periodontal therapy may require reevaluation.