The tragic history of the exposure during the second and third decades of this century in the United States of radium dial workers, patients and members of the public to ionizing radiation from internally deposited isotopes of radium is well documented. Recognition of abnormal health outcomes among female dial workers and determination of a causal association between these outcomes among the workers and their exposure to radium leading to the development of protection standards is a classic example of an epidemiological process whereby knowledge and understanding of "the distribution and determinants of disease" evolve. Health effects studies involving U.S. female dial workers began in the early 1920s and continued into the present decade. These studies are discussed in the context of the epidemiological process whereby cause-effect relationships may be postulated, evaluated and refined to the benefit of workers and the general public.