Abstract The paper presents the results of radon concentration measurements in dwelling houses in Poland’s northeastern region. The investigations were carried out using PICO-RAD carbon detectors. The results from 412 measurements ranged from 4 to 1300 Bq m −3. In the inhabited parts of houses the respective values were: arithmetical mean =25 Bq m −3, geometrical mean =17 Bq m −3 and median =16 Bq m −3. In the basements these values were 76, 36 and 32 Bq m −3, respectively. By taking into account the population density and the mean radon concentration in the inhabited part of the houses, the mean annual dose equivalent from radon was estimated to be 0·63 mSv for the northeastern region of Poland.