A European court told a Spanish Internet provider that companies need not always identify customers that use file-sharing services.
But European Union nations could - if they want to - introduce rules to oblige companies to hand over personal data in similar cases, the European Court of Justice said.
Spain's music rights organization Promusicae had requested Telefonica turn over the names of subscribers that used the file-sharing service KaZaA.
In the ruling, the European court said EU states are not required to reveal customer identities "in order to ensure the effective protection of copyright."
The court did offer copyright-holders could lobby for EU states to change their privacy guidelines to allow disclosure in civil cases.