The patterns of seasonal variation of radon concentrations were measured in 91 homes infive regions of the UK over a period of two years. The results showed that there was nosignificant difference between the regions in the pattern or magnitude of seasonal variationin radon concentrations. The arithmetic mean variation was found to be close to that foundpreviously in the UK national survey. Differences in the pattern between thetwo years of the study were not significant. Two-thirds of homes in the studyfollowed the expected pattern of high radon in the winter and low radon in thesummer. Most of the rest showed little seasonal variation, and a few showed areversed seasonal pattern. The study does not provide any clear evidence for therecorded house characteristics having an effect on the seasonal variation in radonconcentrations in UK homes, though the statistical power for determining such effectsis limited in this study. The magnitude of the seasonal variation varied widelybetween homes. Analysis of the individual results from the homes showed thatbecause of the wide variation in the amount of seasonal variation, applying seasonalcorrection factors to the results of three-month measurements can yield only relativelysmall improvements in the accuracy of estimates of annual mean concentrations.