This paper reports an investigation of concentrations major carcinogenic indoor air pollutants for radon, formaldehyde, and asbestos in the 83 subway stations in the Seoul metropolitan area during November 1991~September 1992. Mean concentrations of indoor pollutants in Seoul subway stations surveyed were 0.23 ppb for formaldehyde, 1.12 pCi/l for radon, and 0.008 fiber/cc for asbestos. Mean formaldehyde concentrations in 83 subway stations were below the U.S. EPA formaldehyde standard (100 ppb), whereas mean concentrations of radon and asbestos in 2% and 22% of total sampled subway stations exceeded the U.S. radon (4 pCi/l) and asbestos (0.01 fiber/cc) standand, respectively. It is likely that possible sources for radon and asbestos are radon intrusion from the leaking underground water and construction materials, respectively.