Objectives - To determine the risk of lung cancer associated with exposure to the decay products of radon, a naturally occurring gas, in homes. Design - Joint analysis of individual data from 13 case-control studies on indoor radon and lung cancer. Setting - Nine European countries. Sample size - 7,148 cases of lung cancer and 14,208 controls. Primary measures - Relative risks of lung cancer and radon gas concentration in occupied homes over the past 5 to 34 years, measured in becquerels (number of radon decays per second) per cubic meter (Bq/m³) of indoor air. Results - The average radon concentration measured in homes of the control group was 97 Bq/m³, of which 11% was >200 and 4% was >400 Bq/m³. In the homes of individuals with lung cancer, the average concentration was 104 Bq/m³. The risk of lung cancer increases by 8.4% (95% confidence interval: 3.0% to 15.8%) for every 100 Bq/m³ increase in measured radon (P=0.0007). This corresponds to a 16% increase (5% to 31%) per 100 Bq/m³ increase in indoor radon—that is, after correcting for the dilution caused by random uncertainties in radon concentration measurements. The dose-response relationship appears to be linear with no lower threshold and remains significant (P=0.04) when the analysis is restricted to individuals living in homes where the radon concentration is <200 Bq/m³. The proportional excess risk does not vary significantly by study, age, sex, or smoking habits. In the absence of other causes of death, the absolute risks of lung cancer at age 75 at typical radon concentrations of 0, 100, and 400 Bq/m³ are approximately 0.4%, 0.5%, and 0.7% for a person who has never smoked, and approximately 25 times higher (10%, 12%, and 16%) for a cigarette smoker. Conclusions - The combined analysis, but not the individual analysis, of these studies indicates that exposure to indoor radon poses a significant risk, particularly among smokers and recent former smokers, and demonstrates that it accounts for approximately 2% of all cancer-related deaths in Europe. (R.A.)