Purpose: The aim of this study was to clarify the aspects of loss of interproximal contact (IC) between fixed implant prostheses and the adjacent teeth. Materials and Methods: A total of 146 implant prostheses in 105 patients were investigated. The loss of IC between fixed implant prostheses and adjacent teeth was evaluated using a 50-µm-thick contact gauge, and the IC was regarded as lost when the gauge was inserted into an IC without resistance at follow-up visits. Statistical analyses were performed to clarify the IC loss rate and factors affecting it. Results: Eighty of 186 ICs (43%) were regarded as lost. The IC loss rate at the mesial aspect was significantly greater than that at the distal aspect (P = .000, Wilcoxon test). Age, the condition of the opposing dentition, the vitality of the adjacent teeth, and the state of splinting of the adjacent teeth affected the loss of IC at the mesial aspect significantly (P .05, Cox regression analysis). Conclusion: Loss of IC between fixed implant prostheses and adjacent teeth was observed frequently at follow-up visits, especially at the mesial aspects of the prostheses. It is considered that this phenomenon may induce food impaction and cause an adverse effect on the peri-implant tissue. Hence, appropriate informed consent and careful maintenance at follow-up visits regarding IC between implants and adjacent teeth are important, and the loss of IC should be recovered if observed.