The week-long anti-competition hearing involving Microsoft and the European Commission has finished - with neither side willing to predict the outcome.
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.
Commission counterpart Cecilio Madero said he wouldn't "speculate about the end result".