The week-long anti-competition hearing involving Microsoft and the European Commission has finished - with neither side willing to predict the outcome.
The 13-judge panel could take months to reach a verdict
Microsoft is appealing against a 2004 ruling when Brussels told it to change how it sells its Media Player software and fined it 497m euros ($613m; £344m).
The Court of First Instance may not deliver a verdict until early 2007.
Defeat could damage the Commission's authority as a competition regulator or Microsoft's future business model.
Microsoft lawyers spent much of the hearing arguing that its fine should be rescinded and that it should not have to produce a version of its Windows operating system without the Media Player software.
They also argued against the 2004 ruling's stipulation that Microsoft should share information about Windows with rival software companies.
The Commission's advocates told the court that the fine was needed as a deterrent and that Microsoft was abusing its market leading position in the desktop software market to stifle competition.
Both sides left the court saying the case was too close to call.
"It would be a mistake to try and predict the outcome and I won't try to do so," said Microsoft lawyer Brad Smith.
Microsoft has come out fighting and wants to protect its secrets
Commission counterpart Cecilio Madero said he wouldn't "speculate about the end result".
When a verdict is eventually produced, Microsoft would still have the chance to make a further appeal to the European Court of Justice.
In the meantime, the Commission is threatening the software giant with fines of up to 2m euros a day if it is found to have delayed its compliance with the anti-competition ruling.
Microsoft has the added headache of the dispute overshadowing one of its most important software launches in years - that of its new operating system, called Vista, and a new suite of business software, Office 2007, due to make a simultaneous consumer debut early next year.