Homesite measurements have been compared with year-long alpha-track measurements of indoor radon in almost 2000 homes in northcentral Virginia and southcentral Maryland. Indoor radon tends to be greater in homes underlain by soil that is more radioactive and more sandy, and in homes that are on hillsides adjacent to larger rivers. Indoor radon tends to be greater during intervals or seasons of greater rainfall. Indoor radon tends to be greater in homes with concrete block walls and in homes with electrical heating systems. In this part of Virginia and Maryland, some communities have over 20% of the homes above 4pCi/L, the level at which the US-EPA says that the home should not be purchased without the installation of radon-reduction equipmnt. In homes that use waterwells for potable water, the average waterborne radon is about 10 times the US-EPA suggested maximum of 300 pCi/L.