In July 2002, FINRA began mandatory dissemination of price and volume information for corporate bond trades. This paper, using recently released data, measures transparency’s effect on trading activity and costs for the entire corporate bond market. Even though trading costs decrease significantly across all types of bonds, trading activity does not increase and, by one measure, decreases. Transparency affects high-yield bonds differently than investment grade bonds. High-yield bonds have the largest decrease in trading activity, 71.1%, and in trading costs, 22.9%. High-yield bonds also disproportionately contribute to the estimated reduction in total trading costs of $600 million a year.