In the memo to Koch Industries employees, Charles Koch said his brother's health has continued to deteriorate.
"His guidance and loyalty, especially in our most troubled times, has been unwavering," the 82-year-old brother wrote.
David Koch also received well wishes from a top executive in the brothers' political-activist network.
"Due to health reasons, David Koch will be resigning from the board of Americans for Prosperity Foundation," said Mark Holden, co-chairman of the Koch-backed Seminar Network. "We greatly appreciate his vital role on the board and all that he has done to help us build a strong foundation for our future success. We wish him and his family well."
Koch was diagnosed with prostate cancer more than two decades ago. Over the years, he and his family foundation, along with other recipients of Koch philanthropic support, have donated millions to battling cancer. He is a board member of the Prostate Cancer Foundation.
He's also been known as a patron of the arts around the New York City area. He gave $100 million to the New York State Theater at Lincoln Center, now known as the David H. Koch Theater. He also donated $65 million to renovate the plaza at the Metropolitan Museum of Art.
The announcement comes a day after the Koch-backed political network unveiled a multimillion-dollar campaign against President Donald Trump's tariffs. The multiyear campaign will include media buys, activist education, grass-roots mobilization, lobbying and policy analysis.
The network has also recently pivoted from focusing its efforts on supporting candidates to legislative initiatives, including protecting recipients of the Deferred Action Childhood Arrivals program and cutting back on government spending.
The Koch network has recently raised eyebrows for its outreach to Democrats on some issues. Last week, Americans for Prosperity published a digital ad thanking Sen. Heidi Heitkamp, D-N.D., for co-sponsoring a bank deregulation bill that eased regulations on mid-sized banks.
Read Charles Koch's memo to employees here:
To: All employees
From: Charles Koch
Subject: David Koch's retirement
In October 2016, David announced that, "Over the summer I was hospitalized and I am now suffering from declining health ..." Unfortunately, these issues have not been resolved and his health has continued to deteriorate. As a result, he is unable to be involved in business and other organizational activities. Because of this, David will be retiring from his responsibilities at Koch and other organizations.
We are deeply saddened by this, as we miss David's insightful questions and his many contributions to Koch Industries. He was instrumental in building our Chemical Technology Group 1,000-fold from a single-product business. His guidance and loyalty, especially in our most troubled times, has been unwavering. David has never wanted anything for himself that he hasn't earned, as his sole desire has always been to contribute. In recognition of his irreplaceable role in building Koch Industries, we are naming him Director Emeritus.
My thoughts of David will always be overflowing with the experiences, challenges, laughter and love of our life together. David epitomizes the requirements for a lasting and beneficial partnership — one with shared vision and values, and complementary capabilities leading to a mutually beneficial relationship.
David has always been a fighter and is dealing with this challenge in the same way. Please respect his situation in your contacts with him.