Previous studies of radon-222 in North Carolina groundwater supplies have shown that the state has some of the highest radon concentrations in the United States. Reanalysis of existing environmental sampling data from 272 public water systems shows that the distribution of radon in North Carolina follows distinct geographical and geological patterns; a simple average concentration based on these samples would not provide a meaningful estimate of public exposure to radon. Variations in radon concentration are associated, in particular, with rock type. The highest radon concentrations in North Carolina groundwater supplies are found in waters from areas in the Piedmont and Blue Ridge regions underlain by granites, and the lowest concentrations (generally < 500 pCi/1) occur in aquifers of the coastal plain. Concentrations in most of the Piedmont region are intermediate (generally between 1000 and 5000 pCi/1). There appears to be no systematic relationship between radon concentration and water system size in North Carolina.