Purpose: The aim of this in vitro study was to evaluate the effect of radiotherapy on the microleakage of three adhesive systems: a one-step self-etching, a two-step self-etching, and an etch-and-rinse system. Materials and Methods: Box-shaped Class V cavity preparations were prepared on the buccal surfaces of 84 extracted human molars. The molars were randomly assigned into 6 groups (n = 14) according to the irradiation and adhesive system as follows: G1, Clearfil S3 Bond (irradiated); G2, Clearfil SE Bond (irradiated); G3, Prime & Bond NT (irradiated); G4, Clearfil S3 Bond (nonirradiated); G5, Clearfil SE Bond (nonirradiated); and G6, Prime & Bond NT (nonirradiated). The cavities were restored with composite resin (Filtek Z 250). After restoration of the samples, a total dose of 60 Gy was delivered in 2 Gy/d fractions for 5 days per week for 6 weeks for the related groups. All specimens were thermocycled for 200 cycles between 5°C and 55°C using a dwell time of 60 s in each bath and then placed in 0.5% basic fuchsin solution for 24 h at room temperature. Specimens were then rinsed and sectioned; the dye penetration at the enamel and dentin margins was examined using a stereomicroscope, and a score of 0 to 3 was assigned. Data were analyzed using the Kruskal-Wallis and Mann-Whitney U-tests. Results: No statistically significant difference existed between the irradiated and nonirradiated groups (p > 0.05). Significant differences in microleakage were observed between enamel and dentin (p 0.01). The microleakage at the dentin margins was greater than at the enamel margins. Prime & Bond NT revealed statistically significantly (p 0.05) higher leakage scores in dentin than did ClearfilS3 Bond and Clearfil SE Bond. Conclusion: Within the limitations of this study, irradiation application did not affect the microleakage of dental adhesive systems. Keywords: dental adhesives, irradiation, radiotherapy, microleakage