The superior alveolar nerves in human adults were investigated macroscopically using whole-mount silver impregnation, paying special attention to their topographical relationship and distribution to the mucous membrane of the maxillary sinus. In addition, the fiber composition of the alveolar nerves was analysed in order to estimate their contribution to teeth innervation. The posterior superior alveolar nerve (PSAN) ran through canaliculi in the lateral wall of the sinus (23 of 37 cases, 62.2%) or under the mucous membrane of the sinus (14 of 37 cases, 37.8%). Moreover, the PSAN gave off many fine twigs to make complex plexus under the mucous membrane of the sinus before joining the superior dental plexus. The plexus of the maxillary sinus was separated from the superior dental plexus by the bony wall of the sinus. After the above gross observations, the perimeter compositions of myelinated fibers of the plexus, PSAN and the anterior alveolar nerve were analysed in the same specimen. The plexus was composed of myelinated fibers of less than 27 microns in circumference perimeter (mean: 14.3 microns) and numerous unmyelinated fibers. The fiber composition suggested that few fibers in the plexus of the maxillary sinus contribute to the formation of the superior dental plexus. The superior dental plexus, innervating the upper teeth, was located in the thick alveolar process of the maxilla, and not on the maxillary sinus wall. The clinical importance of these results was discussed in relation to nerve injury during antrostomy of the maxillary sinus.