Sons of Katie Elder is a fine western, but what elevates it into the classic is the true background story. John Wayne had recently been diagnosed with lung cancer. He had gone through a seven hour operation to remove one of his lungs and part of his stomach. The operation also involved the removal of two of his ribs. He was told to rest for a year, but was committed to filming Sons of Katie Elder. At the time he could walk barely a dozen paces without extra oxygen. To make this even worse Sons of Katie Elder was filmed high in the mountains, where the air was thin.
Despite this Wayne gives his usual bravura performance, convincing the viewer of his immense strenth and stamina.
For those - like me - who are trivia buffs watch out for the scene where, during a fight by a lake, Wayne is pulled in to the water. He almost died in this scene. The water was icy and his remaining lung collapsed. Unable to breathe he was pulled from the water, where he recovered. In early versions of the movie you can still hear a child scream as Wayne plunges into the lake. The scream came from his youngest son, who was watching the filming and knew his dad was very ill.