A radiation accident involving a cesium-137 therapy source occurred in Goiania (Brazil) in September 1987, in which more than 50 individuals were exposed to moderate to high doses (0.2-7 Gy) of gamma-radiation. A cytogenetic technique (i.e., frequencies of dicentrics and rings in peripheral lymphocytes) was employed to estimate the absorbed radiation dose. The follow-up study extending over more than 1 year indicated a decline in the frequencies of dicentrics in the lymphocytes. Using chromosome-specific biotinylated library probes for chromosomes 1, 2, 8 and 19, we studied the frequencies of chromosomal translocations and deletions and the incidence of aneuploidy in the lymphocytes of exposed individuals. In some individuals there was a significant increase in the frequency of translocations and aneuploidy. In other experiments, in which the frequencies of HPRT mutations were determined in lymphocytes using the BrdU-labeling method, some individuals showed an increase (from about 2- to 50-fold) in mutant frequencies.