Purpose: This human case series presents the clinical and histologic results of five cases of peri-implantitis with subsequent sinus graft infections. Complications may follow maxillary sinus augmentation procedures. It is possible to have an inflammatory reaction, movement of the implant inside the sinus, formation of an insufficient quantity of osseous tissue, and the production of an oroantral fistula. Complications following maxillary subantral augmentation procedures are relatively rare; however, the risks and benefits of any surgery must be carefully evaluated at the onset. Materials and Methods: In this case series, bacterial proliferation from infected implants into the grafted biomaterial in sinus cavities was examined. In five cases, removal of infected implants from augmented sinuses did not result in resolution of the infection, but rather in persistence of the infection in the area of the sinus augmentation procedure. Intraoral examination revealed edema/redness in two cases and edema and sinus tract formation in another case. In all cases, surgical curettage of the affected maxillary sinuses was performed. The inserted biomaterials and the accompanying inflammatory tissue infiltrate were totally removed with curettes. The sample was sent for a histopathologic examination. The maxillary sinuses were filled with an autologous platelet gel. Results: Necrotic bone was found lining the different biomaterial grafts. Macrophages were observed around the grafted particles. No blood vessels were observed. Conclusion: This case series is the first to document the spread of infection from an implant surface to the entirety of the graft in the maxillary antrum. Complete removal of all infected bone graft material is the treatment of choice in such cases. Schlagwörter: implant failure, peri-implantitis, sinus failure, sinus graft infection, sinusitis