The aim of the study was to attract attention to the risk of losing a permanent tooth because of an untreated previous primary tooth infection. A 6-year-old girl presented to our clinic with complaints of excessive mobility of a permanent tooth. It was learned from her parents that her upper right primary first molar had broken into pieces because of caries and that she had swelling in her right cheek and had used antibiotics. During an intraoral examination, some pieces of roots of the upper right primary first molar were noticed. The permanent successor tooth was observed erupted as hypoplastic and having excessive mobility. Radiological examination revealed that there was a large radiolucent area around it. The pieces of primary molar’s roots and permanent right upper first premolar tooth were determined to need extraction. In this case report, an upper right permanent first premolar was lost due to infection of the preceding primary molar tooth. Parents must be enlightened as to the importance of primary teeth health and the potential developmental pathology of permanent teeth resulting from infection spreading from the primary tooth.