The estimation and limitation of lung cancer risk from inhaled 222Rn-daughters in mines and in the home is one of the most important problems in radiation protection. The results of epidemiological studies are summarized. The radiation-induced increase in the age-specific lung cancer rate can be interpreted in terms of a proportional hazard model. On the basis of this model, the relative fraction of total lung cancer frequency which might be initiated by the radon-daughter exposure in homes is estimated.