The article discusses effects of seasonal, building, and geological factors on indoor radon-222 (Rn-222) in 312 occupied Tennessee Valley houses. Two-season surveys of 226 houses in Roane County, TN, were conducted in 1985-86, and of 86 houses in Madison County, AL, in 1988-89. Alpha track detectors were placed in each house for 3 months or more during the winter heating season. Detectors were placed at the same sites during the following cooling season. Comparisons were made between winter and summer sampling times and between building types. For the data from Madison County, additional comparisons were made between regions of the county that differed in geological characteristics, especially the thickness of the overburden above the Chattanooga Shale layer, a geological stratum that has high concentrations of radon-226 and is widely found in the southeastern U.S. The geometric means of summer and winter measurements in Roane County were 33 and 54 Bq/cu m, respectively. For Madison County, the means were 121 and 88 Bq/cu m, respectively. The winter Rn-222 concentrations in Roane County exceeded summer concentrations. In Madison County, the opposite was found; atypically, concentrations were higher in the summer.