Since the introduction of radiopaque composite resins, clinicians and researchers have occasionally noticed radiolucent halos adjacent to composite restorations. Such halos have frequently been thought to be recurrent caries or voids in the material. Fifteen of 45 radiopaque composite resin restorations placed by students enrolled in a first-year preclinical operative dentistry course exhibited such halos. Micrographs showed that a thick layer of unfilled resin bonding agent was present. These halos were reproduced in other teeth when excess bonding was not blown off. The results indicate the importance of meticulous technique.