A study with 291 cases and 495 controls on indoor radon and lung cancer incidence was conducted in a Finnish population residing in a high-exposure area. Relative risks of 1.8 and 1.5 for the incidence of lung cancer were observed for those exposed to concentrations of 95-185 Bq m(-3) and 186 Bq m(-3), respectively. The increase in risk was not statistically significant.