Radon concentrations indoors were determined in 24 typical occupied apartments in the city of Thessaloniki, North Greece (40° 38°N, 22° 58°E), by means of 12 surveys, each 2 mo long, over a 2-y period starting October 1989. The ratio of the winter and summer averages for the first year of measurements was 1.8 ± 0.78, and for the second year of measurements was 1.6 ± 0.61. The indoor radon concentration in the summer (1990) period ranged between 8 and 81 Bq m−3, while in the winter (1989–1990) it ranged between 20 and 143 Bq m−3 for the first year of measurements or between 8 and 92 Bq m−3 in the summer (1991) period and between 12 and 119 Bq m−3 in the winter (1990–1991) for the second year of measurements. These results were obtained by type CN-85 alpha track detectors. Some parameters that influenced the concentrations, such as precipitation rates and use of fly ash in building materials, are discussed.