It is known that the incidence of lung cancer is related to inhalation of radon and radon daughters. However, the magnitude of the risk and its dependence upon physiological and environmental factors are still not well defined either experimentally or epidemiologically. Occupational studies of underground miners are the only available human epidemiological information to estimate the risk of exposure to radon daughters in the indoor environment. The results are shown of a study carried out to determine whether lung cancer mortality rates in Spain are significantly correlated with the average indoor radon levels. For this purpose, we have used indoor radon data generated from the national survey carried out in 1989. Lung cancer distribution by cities and deaths, by year of death and sex, were retrieved for each of the different provinces of Spain for the period 1960-1985, showing the evolution and changes in the incidence of lung cancer in the population. Data referring to the evolution of lung cancer for males and females from 1940 until 1985 are also shown. Since cigarette smoking has been linked to lung cancer the effect of smoking habits in the Spanish population was also considered in this analysis. The first results of this study establish no clear evidence of any substantial association between lung cancer mortality rates and indoor radon for males. However, a relationship was evident for females.