Background:There is suggestive evidence that residential radon increases lung cancer risk. To elucidate this association further, we conducted a case-control study in Thuringia and Saxony in Eastern Germany during 1990-1997.Methods:Histologically confirmed lung cancer patients from hospitals and a random sample of population controls matched on age, sex and geographical area were personally interviewed with respect to residential history, smoking, and other risk factors. One-year radon measurements were performed in houses occupied during the 5-35 years prior to the interview. The final analysis included a total of 1,192 cases and 1,640 controls. Odds ratios (OR) and 95% confidence intervals (CI) were estimated by logistic regression.Results:Measurements covered on average 72% of the exposure time window, with mean radon concentrations of 76 Bq/m3among the cases and 74 Bq/m3among the controls. The smoking- and asbestos-adjusted ORs for categories of radon (50-80, 80-140 and >140 Bq/m*3, compared with 0-50 Bq/m3) were 0.95 (CI = 0.77 to 1.18), 1.13 (CI = 0.86 to1.50) and 1.30 (CI = 0.88 to 1.93). The excess relative risk per 100 Bq/mł was 0.08 (CI = −0.03 to 0.20) for all subjects and 0.09 (CI = −0.06 to 0.27) for subjects with complete measurements for all 30 years.Conclusions:Our data indicate a small increase in lung cancer risk as a result of residential radon that is consistent with the findings of previous indoor radon and miner studies.