The seasonal variations of the indoor radon activity concentrations were determined in the 97 dwellings of Trabzon, Turkey. The annual average indoor radon activity concentration varied from 8 to 583 Bq/m(3). The average winter/summer ratio of radon activity concentrations was 3.62. The gamma activity concentrations in the soil samples were determined as 41, 38, 443 and 25 Bq/kg for Ra-226, Th-232, K-40 and Cs-137, respectively. The average gamma dose rate in air and the annual effective dose equivalent for outdoor occupancy were calculated as 63 nGy/h and 77 mu Sv/y, respectively. (C) 2011 Elsevier Ltd. All rights reserved.