Concentrations of the naturally occurring radioactive isotopes radium-226 and radium-228 in excess of the U.S. Environmental Protection Agency standard for drinking water of 5 picocuries per liter have been detected in water from deep aquifers used for public supply that underly parts of northern Illinois. Radium, a known carcinogen, has the potential to cause bone and sinus cancer if ingested in sufficient amounts. This Fact Sheet briefly describes the formation and decay of radium, the health risks associated with radium ingestion, procedures for testing radium concentrations in water, and the occurrence of radium in ground water used for public-water supplies in northern Illinois and provides information on technologies that can reduce the amount of radium in drinking water.