This article analyses how transparency became a buzzword in the European Union (EU) and its predecessors. In order to do so, it examines how the European Parliament (EP), the European Commission, the Court of Justice, and earlier European institutions responded to whistleblowing, between 1957 and 2002. In 2019, the EP agreed to encourage and protect whistleblowers. However, whistleblowing is far from a recent phenomenon. Historical examples include Louis Worms (1957), Stanley Adams (1973), and Paul van Buitenen (1998). Based on policy documents and parliamentary debates, this article studies the attitudes and reactions within European institutions towards whistleblowing. Their responses to unauthorized disclosures show how their views on openness developed from the beginning of European integration. Such cases sparked debate on whether whistleblowers deserved praise for revealing misconduct, or criticism for breaching corporate and political secrecy. In addition, whistleblowing cases urged politicians and officials to discuss how valuable transparency was, and whether the public deserved to be informed. This article adds a historical perspective to the multidisciplinary literature on whistleblowing. Both its focus on the European Coal and Steel Community, European Economic Community, and EU and its focus on changing attitudes towards transparency provide an important contribution to this multidisciplinary field.