Hewlett-Packard said interim Chairman Ralph Whitworth resigned from its board of directors, effective Wednesday, to focus on his health.
Whitworth, who runs activist hedge fund Relational Investors, joined HP's board in 2011. He was appointed HP's interim chairman in April last year.
"While his presence would be a loss, we believe the blueprint for HP has largely been drawn up and believe HP has made meaningful progress in its restructuring and transformation," Wells Fargo Securities analyst Maynard Um wrote in a note.
HP is trying to turn around its fortunes by reducing its reliance on PCs and move toward computing equipment and networking gear for enterprises, to arrest revenue declines and return to growth.
It was not immediately clear what illness Whitworth, 58, was suffering from.
Relational Investors said co-founder Whitworth was taking a leave of absence from the hedge fund and it wished "his family all the best at this difficult time."
"Out of respect for Ralph's privacy, the firm is not providing any additional information," Relational Investors said in a statement.
Relational Investors owned 1.49 percent stake in HP as of March 31 and is among the technology company's top ten shareholders, according to Thomson Reuters data.
HP's board will discuss the appointment of a new chairman at its next meeting, the company said in a statement.
"Given HP's stronger footing and outlook, we believe finding interested and well-qualified candidates that can help guide HP through its next phase should not be overwhelming," Um wrote.
HP's shares were up 0.7 percent at $34.38 in early trading on the New York Stock Exchange on Tuesday. The stock has risen more than 60 percent since Whitworth became interim chairman. (Click here for the latest quote.)