In a geographic correlation study, we explored the possibility that residential exposure to radon in groundwater may be related to cancers other than lung cancer. Measurements of radon in groundwater and 1978-1982 cancer mortality data from North Carolina, USA were used to investigate this relationship. Counties were categorized in two levels of radon exposure according to measured radon concentration and geology. In the lower exposure group (unexposed) county mean radon concentrations ranged from 0-228 pCi/l (0-8436 Bq/m3), and in the upper group (potentially exposed) the range of county average concentrations was 229-10892 pCi/l (8473-403004 Bq/m3) (median 1375 pCi/l (50875 Bq/m3)). Adjusted mortality ratios and 95% confidence intervals were calculated for selected cancers, including leukemias, gastro-intestinal tract cancers, and respiratory tract cancers excluding lung cancer. In contrast to other ecologic studies, we found no consistent association between radon level and cancer mortality.