Studies of radon-exposed underground miners indicate that residential radon is the second leading cause of lung cancer. Seven case-control studies of residential radon have been conducted in North America and two in China, and represent all studies in these areas which included 200 or more lung cancer cases and used long-term radon detectors. North American studies enrolled 4081 cases and 5281 controls, and Chinese studies enrolled 1076 cases and 2015 controls. Based on analyses of pooled data, odds ratios (ORs) and 95% confidence limits at 100 Bq m(-3) were 1.106 (1.00,1.28) for the North American studies and 1.139 (1.01,1.37) for the Chinese studies. Tests of homogeneity of ORs within populations were not significant. Among subjects with complete dosimetry for the 5-30 y exposure period prior to interview, ORs at 100 Bq m(-3) were 1.205 (1.03,1.50) for the North American studies and 1.279 (1.07,0.75) for the Chinese studies. Results are consistent with extrapolations from miners and indicate an excess lung cancer risk from residential radon.