Sinus graft infection is a serious complication that can occur after lateral sinus floor augmentation. Delayed treatment of sinus graft infection can cause rhinitis or paranasal sinusitis. The purpose of this case series was to evaluate the clinical outcomes of immediate and early intra-oral approach for sinus graft infection following lateral sinus floor augmentation with simultaneous implant placement.Seven patients (18 implants) diagnosed with sinus graft infection were included in this case series. Incision and drainage (I&D) were performed immediately (Group I), after one week of antibiotic therapy (Group II), and after two weeks of antibiotic therapy (Group III) after clinical manifestation. Antibiotic treatment continued for 2 weeks after I&D. After two months, infection control and additional treatments including removal of infected graft, removal of implants, detoxification with tetracycline hydrochloride solution, implant replacement, and additional bone grafting procedures were selectively applied depending on contamination of the bone graft material and stability of the implant. Thereafter, uncovering and delivery of the implant prosthesis were accomplished. Implant survival and preservation of sinus graft among the three groups were evaluated clinically and radiographically.After immediate intra-oral infection treatment, sinus graft infection was well controlled, and all clinical symptoms disappeared. Group I showed the best results in implant survival and preservation of sinus graft. However, in group III patients who had two-week antibiotic therapy prior to I&D, improvement of clinical symptoms was most delayed. With the limitations of a case series, we suggest that the timing of intra-oral approach for sinus graft infection is critical for improvement of clinical symptoms and radiographic bone opacity. Also, it seems that I&D performed immediately after clinical manifestation combined with two weeks of antibiotics is the most effective treatment for sinus graft infection.