Lung cancer mortality in a cohort of 4320 miners first employed during 1948-1959 at the Jáchymov and Horní Slavkov uranium mines in West Bohemia and followed until 1 January 1991 has been studied to gain a greater understanding of the consequences of exposure to radon and its progeny. Among men whose exposure rates never exceeded 10 working levels, excess relative risks per unit exposure were greater in younger men, and exposures received in the periods 15-24, 25-34 and 35+ years previously were found to have 47, 24 and 0% of the effect of exposures 5-14 years previously. Within this low-exposure-rate group excess relative risk increased linearly with time-weighted cumulative exposure and did not depend on exposure rate or duration of exposure. For men who spent less than 20% of their employment at the Jáchymov mine the excess relative risk per working level month was 1.36% (95% confidence interval 0.52-3.54) in the baseline category (age group 55+ and exposure received 5-14 years previously). For men who spent more than 20% of their employment at Jáchymov, the corresponding excess relative risk per working level month was higher by a factor of 1.80 (95% confidence interval 1.27-2.97). The difference may be due to the fact that men who spent more than 20% of their employment at Jáchymov were exposed to the much higher levels of arsenic in the dust at the Jáchymov mine than at other mines. When men with exposure rates above 10 working levels were included in the analysis, patterns of risk were complex and depended on both exposure rate and duration of exposure in addition to the factors mentioned above. If these findings are confirmed elsewhere, calculation of risk estimates for extrapolation to modern occupational or environmental exposures should be based on miners with exposure rates below about 10 working levels. Further investigation is desirable of the influence of dusts containing arsenic on lung cancer risk in miners exposed to radon.