David Cook announced the Saturday death at the 12th annual Race for Hope 5K in Washington, D.C., where he was serving as grand marshal. The singer says the cause of death was a brain tumor.
Adam Cook was a lawyer from Terre Haute, Ind., who battled cancer for more than a decade. He famously cheered for his singing sibling in the audience of the popular Fox signing competition's seventh season in 2008.
Julie Henricks, a friend of the Cook family, said Adam Cook died at his Terre Haute home, where hospice nurses were helping Cook's wife, Kendra, care for him.
Henricks said she received a text message Saturday night from Kendra Cook saying that the family had gathered for a bedside vigil as Adam's condition worsened.
She said Adam Cook, an attorney also survived by the couple's 7-year-old son and 6-year-old daughter, had decided to stop his chemotherapy treatments early this year after they failed to help his advanced brain cancer.
"He was so sick and so worn out he just made that decision. That was his choice. It was because he was just so tired," said Henricks.
Adam is the eldest of Cook's brothers. A younger sibling, Andrew, also tried out for the show but didn't make it to Hollywood.
The death was first reported by People.com.
Copyright 2009 The Associated Press.