Dennis Lindberg died Wednesday night at Children's Hospital and Regional Medical Center, his father, Dennis Lindberg Sr., told the Seattle Post-Intelligencer.
Hospital spokeswoman Teri Thomas said she could not confirm or deny anything about the case at the request of the boy's legal guardian, his aunt Dianna Mincin.
Earlier Wednesday, Skagit County Superior Court Judge John Meyer had denied a motion by the state to force the boy to have a blood transfusion. The judge said the eighth-grader knew "he's basically giving himself a death sentence."
"I don't believe Dennis' decision is the result of any coercion. He is mature and understands the consequences of his decision," the judge said during the hearing. "I don't think Dennis is trying to commit suicide. This isn't something Dennis just came upon, and he believes with the transfusion he would be unclean and unworthy."
Doctors had given Dennis a 70 percent chance of surviving the next five years with the transfusions and other treatment, the judge added.
Doctors diagnosed the boy's leukemia in early November. They began chemotherapy at Children's Hospital, but stopped a week ago because his blood count was too low, the Skagit Valley Herald reported. The boy refused the transfusion on religious grounds.
However, his birth parents, Lindberg and Rachel Wherry, who do not have custody and flew from Boise, Idaho, to be at the hearing, believed their son should have had the transfusion and suggested he had been unduly influenced by his aunt, who is also a Jehovah's Witness.
The aunt has declined to talk about the case.
The boy's father told the Post-Intelligencer the ruling shocked him but after visiting his son later Wednesday, he decided not to appeal. He said doctors told him that the boy, unconscious since Tuesday, had likely suffered brain damage.