Sen. John McCain, R-AZ, presumptive Republican presidential candidate gestures during a town hall meeting at the Mizel Family Cultural Arts Center in Denver on May 2, 2008. Sen. McCain concludes his "Call to Action Tour" with his Denver campaign stop. (UPI Photo/Gary C. Caskey)

The Arizona senator had surgery to remove skin cancer after losing the Republican presidential nomination to George W. Bush in 2000. The Stage 2A melanoma McCain had removed is an invasive form of skin cancer that claims the lives of up to 34 percent of those diagnosed within 10 years, Time magazine reported.

When McCain's Navy jet was shot down over Vietnam in 1967, he broke both his arms and his right leg. He was stabbed twice by a bayonet, had his shoulder smashed by a rifle butt and endured five and a half years of captivity -- an experience that left him unable to raise either arm more than 80 degrees.

After his release in Vietnam, McCain was evaluated by Navy psychiatrists who determined he had an "overdeveloped superego" and an "unrealistically high" need for achievement, two characteristics common among presidential candidates, Time said.

If McCain, 71, were to win in November, he would become the oldest first-term president in U.S. history. By comparison, if Democrat Barack Obama is elected, at 46 the Illinois senator would become the fifth-youngest president ever.