Unknown sources of Co-60 radionuclides were recycled and made into more than 20 thousands tons of construction steels in early 1980 in Taiwan and generated more than 300 radiocontaminated buildings since then. However, these buildings were not discovered until late 1992. An epidemiological study was initiated in 1993 to evaluate the health effects on the registered cohort population due to long-term low dose-rate ionizing radiation exposure. The total registered population contained 1,700 persons with more than 5 mSv per year (group I), 2,200 with 1-5 mSv per year (group II), and another 3,000 exposed persons with their dose estimation still pending. Preliminary dose reconstruction was conducted with the collaboration with the NIOSH in 1994. Though results of the comprehensive evaluation are still pending, evidences of adverse health effects have been observed. This talk will describe the preliminary result of this work.