Abstract The purpose of the field trials reported here was to verify the effectiveness of radon protection measures implemented under UK Building Regulations. Measurements were carried out in a total of 423 dwellings using etched track detectors. Some of the dwellings were protected, others were not. Four factors were considered likely to affect the annual average indoor radon level: (1) The radon level of the area. (2) The type of floor construction. (3) The presence or absence of a protective membrane in the floor construction. (4) Whether the dwelling was detached or attached to other dwellings. The radon level of the area is characterised by the proportion of existing houses in the area above the action level. The type of construction is generally in situ concrete floor or block and beam floor. Houses were situated on a total of 33 sites scattered all over Devon and Cornwall in the south west of England (an area of relatively high radon levels) offering a variety of both terrain and housing types. Statistical analysis showed that the effect of the membrane in reducing the radon level was significant at the 0.1% level. The effect of the floor construction, was however only significant at the 5% level. Neither the radon level of the area in which the dwelling was situated nor whether the dwelling was detached or attached had statistically significant effects on the radon level.