Microsoft has lost its appeal against a record 497m euro (£343m; $690m) fine imposed by the European Commission in a long-running competition dispute.
The European Court of First Instance upheld the ruling that Microsoft had abused its dominant market position.
A probe concluded in 2004 that Microsoft was guilty of freezing out rivals in server software and products such as media players.
Microsoft has two months to appeal at the European Court of Justice.
"The Court of First Instance essentially upholds the Commission"s decision finding that Microsoft abused its dominant position," the court"s statement said.
Microsoft"s top lawyer said it was important now for the company to comply with EU competition law, but that it had not yet decided on its next legal steps.