The aim of this study was to compare the accuracy of radiographic diagnosis of secondary carious lesions adjacent to filling materials with different radiopacities. Class II preparations were made in extracted teeth. One group of teeth (n = 49) had caries in the gingival wall, simulating secondary caries. The control teeth (n = 29) had no caries. Three posterior composite materials (P-30 and two experimental composites) with different radiopacities and amalgam were successively inserted into each cavity. After being radiographed, the "restorations" were removed and a new material inserted. The radiographs were interpreted by 11 dentists, and secondary caries was diagnosed according to a five-point confidence rating. The results obtained with the receiver operating characteristics (ROC) method showed that a semi-radiopaque restoration seemed favorable and that the best radiopacity slightly exceeded that of enamel.