The sphincter of Oddi (SO) hepatopancreatic sphincter from 114 Japanese adults, especially the sphincteric muscle mass lying in the duodenal mucosal layer (the submucosal portion of the SO: SMSO), was measured macroscopically under a binocular microscope. The SMSO was classified into two types according to shape. The horizontal type (95/114) usually displayed a rod-like shape (6.0 mm at average diameter at the root) directed toward the anus and was situated on the duodenal muscle wall with the papillary orifice at or near the tip (anal side) of the SMSO. The horizontal type SMSO followed the underlying muscle wall widely ranging from 5.0 mm to 17.9 mm length (9.8 mm at average) and was attached to the wall by the loose connective tissue along the entire length. In the horizontal type, the extramural portion (the portion penetrating and outside of duodenal muscle of the SO) of the SO was very small. The vertical type (19/114) erected on the muscle wall vertically into the lumen of the duodenum. The average length and average maximum diameter of the vertical type were 6.0 mm and 6.2 mm, respectively. The extramural portion of the SO was thicker and tighter in the vertical type, which suggested that endoscopic sphincterotomy might preserve the length of the SO longer than endoscopic papillary balloon dilation (EPBD). Of the individuals in our study, 18% (19/114) had SO diameters ฯฝ5 mm, a finding that may have implications for selection of balloon size in EPBD.