Indoor radon concentrations are subject to seasonal variation with a maximum in winter and a minimum in summer. Procedures to correct for seasonal variation are necessary in order to get an unbiased estimate of the annual average radon concentration from data based on short-period radon measurements. To obtain correction factors, we apply the model developed by Pinel et al (1) to the French database of indoor radon measurements (measurements performed as part of the indoor radon case-control study and of the national radon measurement campaign). For 6-month measurements, the correction factors vary from 0.87 to 1.17 and agree with those previously published. These results might be applicable when assessing indoor radon concentrations in regards to recommended action levels.