The US Environmental Protection Agency intends to establish a maximum contaminant level (MCL) for radon in public drinking water supplies. This article reviews the information on concentrations of radon in US water supplies and the associated risks. It concludes that 1 pCi/L of radon in drinking water produces an average lifetime risk (over 70 years) equal to 4 x 10-7 for radon emanating from the water into the home air and 1 x 10-7 for radon ingested directly with the water. The total risk then is about 5 x 10-7 per pCi/L. Assuming an average radon concentration of about 110 pCi/L in public water supplies, the annual loss of life from waterborne radon is estimated to be about 170, with 95 percent confidence that the true annual loss of life is below 2,700. The concentrations of radon in water corresponding to lifetime risks of 10-4 and 10-3 are 200 pCi/L and 2,000 pCi/L, respectively. Includes 14 references, tables, figures.