Measurements of indoor radon concentrations were performed in 406 residential houses in five counties (Cluj, Bihor, Alba, Bistrita, and Sibiu) using Makrofol and CR-39 alpha-track detectors. From our measurements, an average indoor radon concentration of 82.5 Bq m(-3) for the Transylvanian population was calculated, i.e. an annual effective dose of 2.4 mSv for the whole body. The calculated dose is 62% higher than that previously reported but yet below the recommended action level of ICRP (International Commission on Radiological Protection). A log-normal distribution of the radon concentration was obtained for the studied counties, excluding some higher values from the Stei region--a radon-prone area in Transylvania. For the Stei region, the data show a dual log-normal distribution of the radon concentration with the second maximum being related to the houses built using uranium waste as a construction material. Assuming a cancer risk coefficient of 10(-4)/100 Bq m(-3) year(-1), one can estimate that about 600 lung cancer per year, for about 7,000,000 inhabitants of the Transylvania region, are due to radon inhalation.