[en] Data for natural radiation from a national survey were used to identify areas in Spain with a high level of natural background radiation. Specific measurements were made in these areas to evaluate indoor radon levels as well as the main sources of this natural gas in houses. Values of indoor radon as high as 800 Bq m^-3 were measured and 39% of the houses showed levels over 150 Bq m^-3. For this area, exhalation rates from soil range from 40 Bq m^-2 h^-1 to 900 Bq m^-2 h^-1. Normal vlaues, lower than 10 Bq 1^-1, for radon dissolved in water were also measured. The results are examined and compared with the values obtained in houses located in other areas of Spain. (author)