Read MoreBuffett welcomes shareholders to changing Berkshire
At the beginning of February this year, IBM shares fell as low as $149.52 each, apparently giving Buffett a buying opportunity.
With the stock now trading near $174, IBM is up 8.2 percent since the end of the first quarter on March 31.
As of December 31, Berkshire reported holding 76,971,817 shares. That stake would be worth almost $13.4 billion at Friday's closing price.
It's Berkshire's third largest stock stake, after Wells Fargo at $25.6 billion and Coca-Cola at $16.4 billion.
Read MoreLive Blog: Buffett's annual meeting
You can see all of Berkshire's U.S. stock holdings, as listed in the latest SEC filing, by going to CNBC.com's Berkshire Hathaway Portfolio Tracker.
Buffett first revealed in a November, 2011 CNBC interview that he had been building an IBM stake then worth $10.7 billion. The initial series of purchases, mostly between March and September of that year, were made at an average price of $170 per share.
After answering shareholder questions for five hours today, Buffett will be live with Becky in Omaha for all three hours of "Squawk Box" starting at 6 a.m. ET Monday.