[en] This dissertation describes a cytogenetic evaluation of individuals exposed to cesium-137 gamma radiation during the radioactive accident of Goiania in 1987. The study group is comprised of 14 individuals who had absorbed doses varying from 1 to 7 Gy. In order to obtain metaphase spreads,peripheral blood collected from exposed individuals 1-1,5 year after ionizing radiation exposure, was cultured following basic protocols for PHA-Lymphocyte stimulation. Individual slides were kept for 12 years at room temperature and in the dark. A control group of unexposed individuals was chosen from the Goiania population. Both groups donated blood voluntarily and the unexposed population had no history of exposure to genotoxic agents. The ordinary methodology, consisting of staining chromosomes with Giemsa 4%, was used to determine the biological half-life of unstable chromosomes aberrations. Using Buckston's model (1967) we predicted the half-life of T-cells carrying unstable chromosome aberrations to be between 204-312 days, which is in agreement with the results of Ramalho (1993) who reported a half-life of 140 days a similar for population. Additionally, we used fluorescence in situ hybridization (FISH) as the chosen method to evaluate 12-year old metaphase spreads. To date, we have no knowledge of the use of FISH methodology applied to samples as old as our. Prior to hybridization, chromosomes showed intense dehydration and signs of severe degradation. A pretreatment of aged slides in Carnoy's fixative for 16 hours, followed by exposing the slides to water vapour at 60 deg C for 4 hours improved the fluorescent signal of whole chromosomes probes, making it possible to evaluate old metaphase spreads. Our results suggest that is possible to validate FISH as a viable tool for retrospective biodosimetric studies, even in those cases when chromosome preparations were dehydrated and showed signs of chromosome degradation. (author)