A radiation etiology is well known in thyroid carcinogenesis. RET oncogene rearrangement is the most common oncogenic alteration in Chernobyl-related papillary thyroid cancer (PTC). To find the characteristic alteration associated with RET rearrangements in radiation-induced thyroid cancers, we analyzed the RET oncogene by fluorescence in situ hybridization. The fluorescence in situ hybridization technique has the possibility of detecting RET rearrangements at a single-cell level regardless of the specific fusion partner involved and directly reveals RET copy number on a per-cell basis. Our study demonstrated RET amplification in all 3 cases of radiation-associated thyroid cancers but not in sporadic well-differentiated PTC (n = 11). Furthermore, RET amplification was observed in all 6 cases of sporadic anaplastic thyroid cancers (ATCs). The frequency of RET amplification-positive cells was higher in ATC (7.2%-24.1%) than in PTC (1.5%-2.7%). The highest frequency of RET amplification-positive cells was observed among ATC cases with a strong p53 immunoreactivity. In conclusion, we found RET amplification, which is a rare oncogenic aberration, in thyroid cancer. This report is the first one to suggest the presence of RET amplification in PTC and ATC. RET amplification was correlated with radiation-associated, high-grade malignant potency, and p53 accumulation, suggesting genomic instability. RET amplification might be induced by a high level of genomic instability in connection with progression of thyroid carcinogenesis and, subsequently, be associated with radiation-induced and/or high-grade malignant cases.