Fox was out playing golf on Saturday but felt light-headed so he was admitted to hospital where he will be kept in for observation until he is given the all-clear to return home.
The Broncos said in a statement: "Denver Broncos head coach John Fox was taken to a Charlotte-area hospital after feeling light-headed while playing golf on Saturday. He is being held at the hospital for testing and further observation.
"Our medical staff is currently in contact with Coach Fox, his family and the doctors who are treating him. Additional information will be provided as it becomes available."
Denver are currently 7-1 in their season and are in their bye week.