Bitewing radiographic examination of the Class II composite restorations is commonly performed for diagnosis and preoperative planning of posterior teeth. The purpose of this study was to describe the prevalence; location; and characteristics of radiolucency findings associated with proximal class II composite restorations. Bitewing radiographs of proximal composite restorations of healthy adult patients (18 to 88 years old) who underwent restorative care at predoctoral clinics at the University of Maryland School of Dentistry from August 2016 to July 2019 were identified. Atypical radiographic features were categorized by location and associated material (adhesive or composite). In addition, demographic and clinical details (tooth position; restored surface) were recorded. Out of the 669 radiographically examined bitewings of restorations; 16.5% of radiographs showed no atypical findings; and 83.5% had unusual radiographic signs. The atypical radiographic findings were distributed as 16.5% internal voids; 3% overhang; 7.8% interlayer lines; 12.6% secondary caries; 20.7% interfacial gaps; and 23.1% multiple atypical findings. Class II composite restorations have a high prevalence of atypical radiolucency; particularly in the body of the composite; premolars; and disto-occlusal restorations. The information reported here emphasizes the clinical challenge of ascertaining an appropriate diagnosis of the radiolucencies underneath composite restorations.