This report presents a case with unusual root morphology on both first mandibular molars. Normal root canal anatomy of mandibular first molars is characterised by a birooted tooth with three root canals. A peculiarity that may occur, however, is radix entomolaris (RE). An endodontic retreatment was carried out on the two mandibular molars of a female patient. In both cases, an additional distally located root was found and treated. Consequently, both teeth had three roots, each with four canals. Looking for accessory roots, root canals, and unusual anatomy of the canal system is decisive for a successful root canal treatment. Knowing that they exist enables a successful treatment to be provided, even in complicated cases. Schlagwörter: radix entomolaris, retreatment, three-rooted mandibular first molar