Objectives: To evaluate the efficacy of the combination of platelet-rich fibrin (PRF) and mineralized bone allograft on the volume of the new bone formation in the sinus floor augmentation of the severely atrophic residual alveolar crest with simultaneous dental implant placement. Materials and Methods: All clinical and radiological data of patients treated with sinus floor augmentation using the combination of PRF and demineralized bone allograft with simultaneous dental implant placement between 01/04/2013 and 01/03/2020 were retrieved from the archives. The main inclusion criterion for the study was a residual ridge height less than 2 mm. The volume of the maxillary sinus cavity was measured by a software program on pre-operative and post-operative cone beam computerized tomography (CBCT) sections. The graft volume was calculated by subtracting pre-operative and post-operative total sinus volume. Residual and post-operative ridge heights were measured by the software program on the mid-coronal and mid-sagittal sections of pre-operative and post-operative CBCT images. Results: Eighteen patients with 26 sinus augmentations were included in the study. Forty-six dental implants were placed simultaneously with sinus floor augmentation. The follow-up periods of cases varied between 19 and 29 months. All cases showed new bone regeneration on axial, coronal and sagittal sections of post-operative CBCT data. The mean volume of newly regenerated bone was calculated as 2.1(±0.9) ml. Post-operative ridge heights was significantly increased compared to pre-operative residual ridge heights (p<0.05). There was no sign of a relationship between residual and post-operative ridge heights (p>0.05). Conclusion: The PRF and demineralized bone allograft combination used in sinus floor augmentation may provide healthy new bone that supports simultaneously placed dental implants in function in the severely atrophic alveolar crests with a residual ridge height less than 2 mm. Keywords: Sinüs floor augmentation ; allograft ; cone-beam computed tomography ; dental implant