Many countries are introducing mandatory wage transparency to address the seemingly intractable gender wage gap, but evidence of its effects on gender pay disparities and firm outcomes has, to date, been limited. To examine the benefits and costs of such policies, this column analyses the wages of firms prior to and following the introduction of Denmark’s 2006 Act on Gender Specific Pay Statistics. Mandatory transparency legislation reduced gender pay disparity, primarily by slowing down the growth of men's wages.