Since August 1989, 222Rn groundwater samples from across the state of Arizona have been collected and analyzed using liquid scintillation. Of the 253 specimens acquired, 65% have 222Rn concentrations above 11 Bq L-1 (300 pCi L-1), while 16% have 222Rn activities over 37 Bq L-1 (1,000 pCi L-1). The geometric mean 222Rn concentration for all the wells tested is 13 Bq L-1 x divided by 4; the arithmetic mean is 37 +/- 122 Bq L-1. Using the geometric mean, it is estimated that an additional tracheobronchial lung dose equivalent of 0.19 mSv y-1 x divided by 13.9 is delivered to Arizona residents from the well water to home pathway.