The maxillary floor sinus augmentation is considered as a safe and predictable procedure to ensure optimal implant placement. However, this procedure may have a variety of intra-surgical or post-surgical complications, also the major drawback of the procedure is deemed maxillary sinusitis. This case is a very unusual delayed occurrence of acute maxillary sinusitis after simultaneous maxillary sinus augmentation, using xenograft and implant placement. This report describes a serious complication of the maxillary sinus augmentation.