Radon concentrations indoors were determined in 24 typical occupied apartments in the city of Thessaloniki, North Greece (40o 38'N, 22o 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.