Sinus membrane perforation arising during the sinus lift procedure is a very severe complication that can induce the failure of vital bone formation in the bone-graft area, the failure of dental implant osseointegration and sinusitis. In most cases, the size of the perforation was small; therefore, various techniques can be applied to treat sinus membrane perforation. However, the variability in anatomy of the sinus cavity and related surgical miscalculations frequently result in tearing of the Schneiderian membrane. In the case of and extremely thin maxillary sinus membrane, achieving a membrane lift that does not involve membrane tearing is difficult. Sometimes, a pre-existing membrane perforation must be repaired prior to implant restoration; however, in this case, the perforated area may be too large to manage simply. In this report, two cases in which large perforations of the maxillary sinus were repaired through the lateral window approach are reviewed. Pre-existing oro-antral perforations were observed in both cases. In one case, the perforation area was so large that the torn membrane of the maxillary sinus was sealed and supported with a bilayered collagen membrane. In the other case, the size of the oro-antral perforation was extremely large, and the destruction of the sinus bony floor was severe. Block bone grafting and a collagen membrane were used for support and sealing of the torn membrane and for the bone supporting maxillary sinus floor. The successful repair of sinus membrane perforation was verified by radiographic examination, and a clinically good prognosis was observed during follow-up period. (JOURNAL OF DENTAL IMPLANT RESEARCH 2012;31(3):73-78)