Pituitary adenomas can expand upward through the foramen diaphragma sellae (FDS), compress the visual pathways on the suprasellar region, and cause diverse visual defects. However, the relationship between the FDS and the visual pathway has not been thoroughly clarified. This study was thus performed to determine the topographic relationship between these two structures. One hundred heads of adult cadavers were examined in this study. The FDS was classified into five types (Ia, Ib, Ic, II, and III) according to its location relative to the four parts of suprasellar region of the visual pathways. The midpoint of the optic chiasm (OC) was located on the midline passing through the crista galli in 70% of cases, but to the left and right sides of the midline in 9 and 21% of cases, respectively. The FDS was completely covered by OC in 30% of the cases, but it was partly seen superiorly in 70%. The pituitary infundibulum passed mainly through the center middle or posterior middle part of nine partitions of the FDS. The horizontal and vertical diameters of the foramen were 7.9 +/- 2.0 and 7.6 +/- 1.9 mm, respectively. The length of the optic nerve was 9.7 +/- 1.9 mm on the left side and 9.5 +/- 1.9 mm on the right side. The angle between the optic nerve and the midline was 34.5 degrees +/- 5.7 degrees on the left side and 36.0 degrees +/- 6.3 degrees on the right side. The results of this study are expected to further the current knowledge of the topographic anatomy on suprasellar structures.