OBJECTIVES: To assess the level of transparency of health technology assessment (HTA) organizations in eight countries (Australia, Canada, Brazil, France, Germany, Italy, Spain, and the United Kingdom). METHODS: Nine national review bodies carrying out HTAs for the purposes of reimbursement and funding decisions within their respective countries were assessed according to 33 transparency parameters. The parameters were designed to assess the level of transparency in the HTA processes, clinical and economic reviews, and stakeholder involvement.  Using a modified Delphi process, each parameter was weighted based on its importance to transparency and given a score from one to ten. A review of each national body was conducted to determine whether or not they included each of the 33 parameters. Data from public sources (i.e., organization websites) were utilized and interviews with international experts were conducted to verify the information. Total scores were calculated and converted to a transparency index, with 100% being the most transparent. The score was adjusted for review bodies that did not require economic evaluations for their HTA process. RESULTS: The level of transparency varies greatly amongst the HTA organizations studied.  The transparency index scores were as follows: Institut für Qualität und Wirtschaftlichkeit im Gesundheitswesen/Gemeinsamen Bundesausschusses (Germany), National Institute for Health and Care Excellence (United Kingdom), pan-Canadian Oncology Drug Review (Canada), Common Drug Review (Canada), Pharmaceutical Benefits Advisory Committee (Australia), Comissão Nacional de Incorporação de Tecnologias (Brazil), Haute Autorité de Santé (France), Agencia de Evaluación de Tecnologias Sanitarias (Spain), and Agenzia Italiana del Farmaco (Italy) were 97%, 96%, 91%, 83%, 78%, 70%, 67%, 53%, and 25%, respectively.  CONCLUSIONS: Transparency amongst HTA organizations is progressively becoming the international standard. However, the extent of transparent processes and procedures proves to be heterogeneous amongst international review organizations.