In July 1973, a study at the University of Chicago linked radiation treatment during childhood to a variety of diseases, including thyroid cancer. A few months later, a worker at Michael Reese Hospital in Chicago, Illinois found a registry of 5266 former patients who had been treated with radiation during the 1950s and 1960s. Hospital officials decided to contact these patients and arrange for follow-up medical examinations. Media coverage of the hospital's campaign had a snowball effect that prompted more medical institutions to follow suit, resulting in the National Cancer Institute (NCI) launching a nationwide campaign to warn the public and medical community about the late health effects of ionizing radiation. This study describes how the single action of a hospital in Chicago and the media attention it attracted led to a national campaign to warn those who underwent radiation treatment during childhood.