The Union Carbide Corporation and Umetco Minerals Corporation (defendants) ran a uranium and vanadium mining operation in Colorado. The defendants founded a town called Uravan to house their employees. From 1936 until the end of operations in 1984, the defendants produced 42 million pounds of uranium oxide. By 1986, the last residents of Uravan had been evacuated, because Uravan had been identified by the Environmental Protection Agency as a hazardous site requiring remediation. Twenty-seven former residents of Uravan (plaintiffs) who had been diagnosed with various cancers or thyroid disease sued the defendants for negligence. At trial, the defendants argued that the plaintiffs had failed to provide evidence that their injuries would not have resulted but for the defendants’ conduct. The plaintiffs argued that the law only required them to show that the defendants’ radiation emissions were a substantial factor in their injuries. The district court agreed that Colorado law required the plaintiffs to show but-for causation, which they failed to do. Consequently, the court granted summary judgment in favor of the defendants. The plaintiffs appealed.