Warren Buffett surprised the investing world this week when he broke his longstanding tradition of staying away from technology stocks, disclosing a 5.4 percent stake in IBM and a smaller investment in Intel.

Why not Microsoft? It’s an interesting question given that Buffett probably knows more about the Redmond company than about any other tech stock, given his longtime friendship with Bill Gates. Not to mention that epic 1997 email to Buffett from then-Microsoft executive Jeff Raikes (which later surfaced in an antitrust case) making the theoretical case for the value of an investment in the software company.

In fact, Buffett said on CNBC this week that his friendship with Gates is the reason he didn’t buy Microsoft stock.

Microsoft is a special case because Microsoft is off bounds to us because of my friendship with Bill and if we spent seven months buying Microsoft stock and during that period they announced a repurchase or increase of the dividend or an acquisition, people would say you’ve been getting inside information from Bill. So I have told (investment gurus) Todd and Ted and I apply it myself that we do not ever buy a share of Microsoft. I think Microsoft is attractive but … we will never buy Microsoft. … People would just assume I knew something and I don’t, but they would assume it and they would assume Bill talked to me and he wouldn’t have. But there’s no sense putting yourself in that position.

Whether it’s a legitimate reason or a convenient excuse, it sounds pretty definitive.

Earlier: Gates: Microsoft’s opportunity ‘stronger than it’s ever been’ … Live notes from Microsoft’s annual shareholder meeting.

Photo by Asa Mathat/Fortune via Flickr

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