Ionizing radiation is a well-known risk factor for thyroid cancer in human populations. Chromosomal rearrangements involving the RET gene, known as RET/PTC, are prevalent in thyroid papillary carcinomas from patients with radiation history. We studied the generation of RET/PTC in HTori-3 immortalized human thyroid cells exposed to a range of doses of γ-radiation and harvested 2, 5–6, and 9 d later. RET/PTC1 and RET/PTC3 were detected by RT-PCR followed by Southern blotting and hybridization with internal oligonucleotide probes. No RET/PTC was found in cells harvested 2 and 5–6 d after irradiation, whereas 59 RET/PTC events were detected in cells collected 9 d after exposure. The average rate of RET/PTC induction was 0.1 × 10⁻⁶ after exposure to 0.1 Gy, 1.6 × 10⁻⁶ after 1 Gy, 3.0 × 10⁻⁶ after 5 Gy, and 0.9 × 10⁻⁶ after 10 Gy. When adjusted for cell survival, the rate after 10 Gy was comparable with those after 5 Gy. RET/PTC1 was more common than RET/PTC3 after each dose, comprising 80% of all rearrangements. In this study, we demonstrate a dose-dependent induction of RET/PTC rearrangements in human thyroid cells after exposure to 0.1–10 Gy γ-radiation. This provides additional evidence for a direct link between this genetic event and radiation exposure and offers a powerful experimental system for studying radiation-induced carcinogenesis in the thyroid gland.