Health care providers rely on group purchasing organizations (GPO) to negotiate contracts with vendors of medical products. In 2002, questions were raised about GPOs engaging in potentially anticompetitive business practices such as collecting excessively high contract administrative fees. In 2003, GAO reported that selected GPOs had adopted or revised codes of conduct to respond to the questions about their business practices, but that it was too soon to evaluate the impact of the codes of conduct. GAO was asked to provide information on GPOs. In this report, GAO describes (1) the types of services that GPOs provide and how the GPOs fund these services, (2) initiatives that GPOs have implemented since 2002 to address the questions that had been raised about their business practices, and (3) the reported impact of the GPOs' codes of conduct and other initiatives. To do its work, GAO reviewed GPO documents and collected written responses to structured questions from the six largest GPOs based on their reported 2007 purchasing volume. GAO also conducted follow-up interviews with these six GPOs. GAO interviewed representatives from six GPO customers--hospitals--that varied in size, the GPOs with which they did business, and whether they had an ownership stake in a GPO. GAO also interviewed five medical product vendors of various sizes that do business with GPOs.