Approximately 58,000 indoor radon measurements are available for homes in Maryland. A comparative study between compilations of activated-charcoal and alpha-track measurements of indoor radon in zip-code-size geographic areas indi cated that both of these methods are useful and are equally able to estimate regional indoor radon. Indoor radon measurements compiled according to zip code areas can be used to create state-size radon hazard maps. In Maryland the area with the highest indoor radon (mostly composed of zip code areas that average over 8 pCi/L) is the western half of the Piedmont Province and the eastern side of the Coastal Plain Province. The eastern half of the Piedmont and the eastern half of the Valley and Ridge mostly have intermediate and high indoor radon levels (4-8 and > 8 pCi/L). The Blue Ridge, western side of the Valley and Ridge, and Plateau Province each has relatively few zip code areas, but the data suggest a range from low to high indoor radon levels. The western side of the Coastal Plain has the lowest indoor radon (most of the zip code areas average less than 4 pCi/L).