Radon-222 gas arises from the radioactive decay of radium-226 and has a 3.8 day half-life. This gas percolates up through soil into buildings, and if it is not evacuated there can be much higher exposure levels indoors than outdoors, which is where human exposure occurs. Radon exposure is classified as a human carcinogen and new Danish homes must be constructed to ensure indoor radon levels below 100 Bq/m(3) . Our purpose was to assess how well 200 newly constructed single-detached homes perform according to building regulations pertaining to radon and identify the association between indoor radon in these homes and municipality, home age, floor area, floor level, basement and outer wall and roof construction. Median (5-95 percentile) indoor radon levels were 36.8 (9.0 - 118) Bq/m(3) , but indoor radon exceeded 100 Bq/m(3) in fourteen of these new homes. The investigated variables explained nine percent of the variation in indoor radon levels and although associations were positive none of these were statistically significant. In the present study radon levels were generally low, but we found that fourteen (seven percent) of the 200 new homes had indoor radon levels over 100 Bq/m(3) . More work is needed to determine the determinants of indoor radon. © 2012 John Wiley & Sons A/S. Published by Blackwell Publishing Ltd.