According to the document, filed last week, Microsoft divested its remaining stake in Comcast sometime before Dec. 31. At one point, Microsoft held about 150 million Comcast shares, which on Tuesday closed at $14.02 -- about 40% off its 52-week high.
Microsoft announced a high-profile, $1 billion investment in Comcast in 1997, with an eye to helping the company build out its high-speed data and video services.
"Our vision for connecting the world of PCs and TVs has long included advanced broadband capabilities to deliver video, data, and interactivity to the home," Microsoft chairman Bill Gates said at the time. "Comcast's integrated approach to cable distribution, programming, and telecommunications complements that vision of linking PCs and TVs."
In recent months, Comcast has drawn heavy criticism from the Federal Communications Commission and Internet advocates for a decision to block bandwidth-hungry peer-to-peer services from its networks. It's since rescinded the practice.
It was not immediately clear if Microsoft's selloff of Comcast shares is related to the controversy.
It's also possible that Microsoft is looking to raise cash in advance of an acquisition, such as another attempt to buy out rival Yahoo's search business.
Carol Bartz, who replaced Jerry Yang as Yahoo's CEO last week, has reportedly told company employees that she would investigate whether a sale of Yahoo's search business makes sense. She has also reportedly met informally with Microsoft CEO Steve Ballmer.
Microsoft shares were up slightly more than 1%, to $18.68, in pre-market trading Wednesday.