John Wayne died from nuclear fallout. Really.

In 1955, John Wayne and a film crew of 220 people set out into the Utah desert to film The Conqueror. While the fact that such a movie was made is tragic--the IMDB plot description reads: Mongol chief Temujin battles against Tartar armies and for the love of the Tartar princess Bortai. Temujin becomes the emperor Genghis Khan and is, of course played by the very white John Wayne--what's more tragic is that the set was in a nuclear fallout zone.

Of the crew, nearly half contracted cancer, and 46 died from it, including John Wayne himself. Was nuclear fallout the only ominous product of this Howard Hughes film? No. A black panther also tried to eat the lead actress. From the folks at The Straight Dope:

The movie was shot in the canyonlands around the Utah town of St. George. Filming was chaotic. The actors suffered in 120 degree heat, a black panther attempted to take a bite out of Susan Hayward, and a flash flood at one point just missed wiping out everybody. But the worst didn't become apparent until long afterward. In 1953, the military had tested 11 atomic bombs at Yucca Flats, Nevada, which resulted in immense clouds of fallout floating downwind. Much of the deadly dust funneled into Snow Canyon, Utah, where a lot of The Conqueror was shot. The actors and crew were exposed to the stuff for 13 weeks, no doubt inhaling a fair amount of it in the process, and Hughes later shipped 60 tons of hot dirt back to Hollywood to use on a set for retakes, thus making things even worse.

Many people involved in the production knew about the radiation (there's a picture of Wayne himself operating a Geiger counter during the filming), but no one took the threat seriously at the time. Thirty years later, however, half the residents of St. George had contracted cancer, and veterans of the production began to realize they were in trouble. Actor Pedro Armendariz developed cancer of the kidney only four years after the movie was completed, and later shot himself when he learned his condition was terminal.

Howard Hughes was said to have felt "guilty as hell" about the whole affair, although as far as I can tell it never occurred to anyone to sue him. For various reasons he withdrew The Conqueror from circulation, and for years thereafter the only person who saw it was Hughes himself, who screened it night after night during his paranoid last years.

The Straight Dope is a great website. Check it out!