Warren Buffett's company bought nearly 3.7 million more shares of DaVita Inc. after the dialysis provider reported its earnings last week.
Berkshire Hathaway Inc. said in documents filed with the Securities and Exchange Commission on Friday that it now owns 35.1 million shares of DaVita.
That means Berkshire now owns about 16.5 percent of DaVita, but it has agreed not to buy more than 25 percent of the Denver-based company unless, both companies agree.
Denver-based DaVita runs nearly 2,000 outpatient dialysis clinics.
DaVita said last week that its operating costs increased during the quarter, and it said the Justice Department has been investigating its business practices.
The stock was trading above $56, but quickly lost more than $2 a share after DaVita's earnings report. That's when Berkshire started buying.