We performed measurements in 16 houses (12 concrete-slab houses and 4 brick houses) and within each house we performed measurements in one or more rooms from cellar to 4th floor. In total, the study includes 279 indoor radon measurements. The method of alpha track detectors was used in this investigation. The measurements were performed in housing of the same construction of concrete slabs in one part of Białystok. Approximately 12 million people in Poland live in such concrete slab buildings. It was observed that mean radon concentrations in brick houses were higher (by 24%) than in concrete slab buildings situated in the same housing estate. The results for the Green Hills housing estate are believed to be representative for all concrete slab buildings in northeastern Poland.