Eight cases of medial blowout fracture underwent endoscopic surgery by a transnasal approach, and the cause of double vision was investigated by analyzing CT and operative findings. The medial blowout fractures were classified into three groups by CT findings; comminuted fracture type, adhesion fracture type and trapped door fracture type. The fractures were also classified into three groups on the basis of operative findings: strangulated type, adhesive type and obstruction of sinus type. Each group delineated on the basis of operative findings corresponded to a CT findings group: comminuted fracture type and strangulated type, adhesion fracture type and adhesive type, trapped door fracture type and obstruction of sinus type. The grade of double vision was the most serious with the comminuted fracture type, followed by the adhesion fracture type and trapped door fracture type. These results suggest that the cause and grade of double vision can be determined before surgery using CT findings, and this information may help in the operation for double vision. Since endoscopic surgery for medial blowout fracture is not complicated, the trapped door fracture type as well as the comminuted type of fracture were considered to be indicative of surgery.