Radiation-related caries is a disease with high potential of teeth destruction in patients who have undergone radiotherapy in the head and neck region. Also, it is a challenge for dentistry due to high rates of recurrent caries and early restorations failure. This study aims to analyze the early restoration failures in order to better understand the etiology of dental restorations reduced longevity in irradiated teeth. Fifteen restored permanent teeth extracted from 11 patients who had finished head and neck radiotherapy were studied. Sections from each tooth were prepared and a qualitative description of the interface between restorations and dentin was performed by using polarized light microscopy and scanning electron microscopy. Unfavorable anatomical shape of restorations, residual caries, and secondary caries affecting dentin adjacent to restorative materials were widely found. The morphological patterns of these carious lesions were similar to conventional dentin lesions with superficial demineralized zone and translucent zone. Early dental restoration failure in teeth affected by radiation-related caries may have the same etiological factors from ordinary dental restoration failure and direct radiogenic damage to dentition would not be essential to early restorations failure in radiation-related caries.1883-7