There is no definitive answer to this question as the percentage of cats that run away and die varies depending on a variety of factors, including the size and age of the cat, whether or not it has been vaccinated against diseases like feline leukemia, and whether or not it is living in an environment where there are other cats. However, according to one study published in The Journal of Feline Medicine & Surgery, approximately 30 percent of cats that run away end up dying from causes related to their running behavior.