A Dutch judge has blocked attempts to find out who may be illegally sharing movies and music in Holland.
The identity of Dutch file-sharers has been protected
Dutch industry group Brein was behind the attempt to discover the identities of file-sharers following investigative work by a US firm.
But the judge said the US company had broken Dutch privacy laws which meant net service firms did not have to hand over data about name and addresses.
Anti-piracy group Brein said it would take the case to a higher court.
Five Dutch net service suppliers - UPC, Essent Kabelcom, Tiscali, Wanadoo and KPN - defied demands by Brein to hand over the names and addresses of customers.
Brein made the demands following work by US firm Media Sentry to identify the net addresses, or IP numbers, of customers illegally swapping copyrighted music and movies via file-sharing networks.
The court in the Netherlands said that in finding out the net addresses Media Sentry had probably broken tough Dutch laws that protect privacy.
Christiaan Alberdingk Thijm, the attorney representing the five net suppliers, said the ruling was "an important victory for the privacy of internet users".
But it might only signal a short reprieve for the file-sharers because the Utrecht court said it did have the power to force net firms to surrender data if Brein found out the information legally.
The anti-piracy organisation has said that it would try again to get hold of the identification data.
Already nine of the people found and identified by Brein have reached out of court agreements with the group.
Brein represents 52 media organisations including EMI, Universal and Sony Music.