Restoring the proximal contacts is important for a restoration's long-term success. A frequently observed late complication of implant restoration is proximal contact loss. At present, there is a lack of sufficient research for determining the prevalence of proximal contact loss and for identifying the causative factors. The purpose of this clinical retrospective study was to evaluate the prevalence of proximal contact loss between implant restorations and adjacent teeth and to identify the causative factors. Partially edentulous patients who had received single crowns or fixed dental prostheses on implants were selected; the rehabilitations were metal-ceramic or all-ceramic, screw- or cement-retained. The primary study outcome was clinical evaluation of mesial and distal proximal contact tightness. The secondary outcome was evaluation of patient awareness of proximal contact loss, food impaction, and occurrence of biologic complications. In total, 237 single crowns and 83 fixed partial dentures were assessed. A multivariate logistic regression model was adopted. The overall prevalence of proximal contact loss was 51%. Among the patients with proximal contact loss, 107 (65%) were aware of its presence, while 58 (35%) reported food impaction. Within the limits of the present study, proximal contact loss between implant prostheses and adjacent teeth can be considered a frequent event, even at a 10-year follow-up, that should be carefully considered and monitored by patients and operators.