According to The Associated Press:
Thomas was hospitalized with pneumonia just before the 2006 election, but won with 70 percent of the vote, monitoring the election from his hospital bed.
Two days after the election, Thomas announced that he had just been diagnosed with leukemia.
President Bush called Thomas "a man of character and integrity known for his devotion to the values he shared with the people of Wyoming."
"He leaves a lasting legacy as a guardian of Wyoming's lands and resources and our country's National Parks," Bush said.
Gov. Dave Freudenthal, a Democrat, will appoint a successor from one of three finalists chosen.