The effectiveness of remedial measures in houses with high radon levels has been tested in 943 houses in the UK. Radon levels were measured for 3 mo before and after remediation, and the results were corrected for typical seasonal variations. Householders completed questionnaires about house characteristics and remedial measures. The results, were analyzed to determine the influence of house characteristics on the effectiveness of different remedial measures. Significant differences in effectiveness were found, in particular depending on the age of the house and whether the measures were installed by a major contractor, a local builder, or the householder.