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BACKGROUND The literature is scanty regarding the effect of radiation therapy (RT) on the mechanical properties of immature permanent teeth. AIM To evaluate the effect of RT on the fracture resistance of simulated immature teeth submitted to different types of root reinforcement. DESIGN Sixty-four human teeth simulating the Cvek stage 3 of root development were distributed into 8 groups (n=8), according to exposure or not to RT (70 Gy) and the root reinforcement method: Group NR (control) - no reinforcement/no RT; Group NR+RT (control) - no reinforcement/RT; Group PO - tricalcium silicate-based cement (TS) apical plug/canal obturation/no RT; Group PO+RT - TS apical plug/ canal obturation/RT; Group TS - canal filling with TS/no RT; Group TS+RT - canal filling with TS/RT; Group FP - TS apical plug/fibreglass post/no RT; and Group FP+RT - TS apical plug/fibreglass post/RT. Fracture resistance was determined using a universal testing machine (0.5 mm/min). RESULTS In the intergroup comparison, non-irradiated teeth had higher fracture resistance (p<0.05). Groups FP and FP+RT had higher fracture resistance (p<0.001). CONCLUSION Radiotherapy affected the fracture resistance of simulated immature teeth. Reinforcement with fibreglass posts increased the fracture resistance, regardless of the radiation.