Purpose: The main aim of this study was to investigate the loss of maxillary teeth opposing mandibular implant-supported prostheses (ISPs) placed at dental clinics. Materials and Methods: Participating dentists were asked to provide information on implant procedures they had undertaken between January 1, 1990, and December 31, 2009. Data were requested for procedures in which implants were inserted into only one side of the mandible in posterior regions and no implants were inserted in other regions. Results: Data on a total of 383 patients were analyzed. Participants in this study consisted of 170 men and 213 women ranging from 20 to 78 years of age (mean age, 51.0 years). Natural teeth were divided into three groups: opposing teeth (teeth opposing ISPs), control teeth (teeth of the same type as the opposing teeth but on the other side of the maxilla), and other teeth. The mean observation period was 72 months. One hundred six teeth were lost during the observation period. Fourteen opposing teeth were lost (1.72%, 14/816), 15 control teeth were lost (1.84%, 15/816), and 77 other teeth were lost (0.98%, 77/7,891). The percentage of opposing teeth lost was significantly higher than that of other teeth. However, there was no difference in the rates of loss of opposing and control teeth. Conclusion: ISPs in posterior mandibular regions are not a risk factor for the loss of opposing teeth. Keywords: clinical study, dental implants, implant-supported prostheses, opposing teeth, tooth loss