[en] A survey of radon in Danish dwellings has been carried out. The concentration of radon-222 has been measured in 3019 single-family houses and 101 multi-family houses (apartment complexes) with the so-called alpha-track technique (CR-39). On this basis, a map has been established. This map shows the percentage of single-family houses in each of the 275 municipalities with levels above 200 Bq/m³ (i.e.) the action level recommended by the Danish building authorities for simple radon remedi-action). For Denmark as a whole, 4.6 % of the single-family houses are above 200 Bq/m³. That corresponds to 65 000 houses. In certain parts of the country (e.g. northern Jutland) less than 1 % are above 200 Bq/m³. In other places (e.g. certain parts of Funen and Bornholm) the value is above 10 %. In the survey, 15 houses had levels above 400 Bq/m³. The relationship between radon and various factors has been investigated with regression analysis. The three most important factors are type of basement, province, and soil type. The analysis confirms that the soil below the houses is the most important source of radon in Danish single-family houses. The study also provides a more detailed picture of the relation between geology and indoor radon. For example, it is shown that clayey till generates different indoor radon levels in the different parts of the country. The study does not indicate that a substantial part of newer Danish houses have an airtight house-soil interface. The report includes a summary in English. (au)