Radon is a naturally occurring, colourless and odourless radioactive gas, with a halflife of 3.8 days. Dangerous is not radon itself, but its progeny (daughter) products. The radon decay products irradiating of the lung. There is a strong correlation between radon exposure and lung cancer. The indoor radon concentrations in residential buildings are monitored. The study is performed in eight storied blocks of flats and also in family houses. The contributed deals with the radon concentration depending on a year season and floor level. Radon concentrations are varying with the characteristics of the building and its ventilation. Statistical analysis of the results shows that the probability that a weekly measurement represents the yearly radon concentration is significantly higher (twice) in winter and in summer than in summer and autumn. The maximum values of radon concentration is obtained in the first and second-floor rooms might be due to the contribution of radon emanation from the soil, entering into the living spaces from the ground.