This study represents the first statewide random sampling of private water-borne radon-222 in the nation, and also collected information on radium-226 and radium-228. The health risks of water-borne radon-222 are considered high, and the U.S. EPA has proposed a maximum contaminant level (MCL) of 300 pCi/ L for public water supplies. Study findings include: 1) Participant collected samples at the point-of-use are comparable to samples professionally collected; 2) Fifty-two percent of the wells sampled had waterborne radon-222 concentrations that exceeded the proposed MCL; 3) Radon-222 concentrations in private well-water are slightly higher than groundwater supplies. (This is attributed to a higher percentage of private wells drilled into Pleistocene till which often produce higher waterborne radon-222 concentrations, while public supplies are preferentially drilled into alluvial deposit, which generally contain less radon-222 precursor material); 4) The western part of the state has the highest mean radon-222 well-water concentrations as a result of the large number of wells drilled into the Pleistocene; 5) Radon-222 samples did not vary temporally during the one year collection period; 6) The contribution of well-water derived indoor air radon-222 is minimal compared to subsurface soil and rock radon-222 sources in Iowa; and 7) Well depth, well-water radium-226, well-water radium-228, and indoor screening of radon-222 are all extremely poor predictors of well-water radon-222 concentrations. Additionally, because of the huge variations of waterborne radon-222 concentrations noted for the specific aquifer type, their usefulness as a predictor of waterborne radon-222 concentration is limited.