[en] Investigations on underground miners show an increased risk of lung cancer associated with exposure to radon and its decay products. Epidemiological studies on indoor exposure to radon progeny partly prove an increase of lung cancer as well, although the results are inconsistent and not yet sufficient for quantitative risk assessment. Based upon low dose extrapolation the Commission on Radiation Protection (Germany) estimates a 4-12% portion of the total lung cancer cases in Germany, to be caused by radon. Applied to a total annual amount of about 25,000 lung cancer deaths in the western part of Germany, this theoretical estimate corresponds to an annual number of 1000-3000 lung cancer deaths. For a reliable risk assessment based on empirical evidence specific and sufficiently large epidemiological investigations are required. A case-control study of more than 3000 cases and the same number of controls has started in several regions of Germany. This report describes the study design and first experiences. (author)