Dentin dysplasia (DD) is a rare developmental dentin disorder that causes root malformation. It is divided into radicular DD type 1 (DD-1) and coronal DD type 2 (DD-2). Recently, a new entity causing localized root malformation of permanent first molars that resembles DD-1b has been described as molar-incisor malformation (MIM). We report and compare 4 new cases that exhibit similar clinical, histologic, and radiographic features to the new entity, MIM. We believe MIM and our 4 cases to be the same entity, which is nonhereditary and, because of the isolated but bilaterally symmetric pattern of involvement, may be caused by a short-duration environmental insult that disrupts normal development/function of Hertwig's epithelial root sheath. We propose the name symmetrical multiquadrant isolated dentin dysplasia as the most appropriate descriptive designation for this unusual but highly distinctive anomaly.