The causes of idiopathic exophthalmos were still unknown. We used computed tomography to compare the orbital structures of patients with idiopathic exophthalmos and those of a healthy Japanese population. A total of 18 orbits (ranged 17-52 years) in idiopathic exophthalmos and 334 normal orbits (ranged 17-59 years) were included. On computed tomography, the eyeball and orbit sizes, lateral wall thickness and length, perpendicular distance from the interzygomatic line to the medial orbital rim, and the nasal cavity width were measured. No significant differences in eyeball or orbit sizes were found between the idiopathic exophthalmos and control groups (P > 0.05). In contrast, the mean lateral wall thickness of the idiopathic exophthalmos group was significantly thicker than that of the control group (P < 0.01). The lateral wall length was shorter with significant difference (P < 0.05). The perpendicular distance and the width of the nasal cavity in the idiopathic exophthalmos group were longer with significant difference (P < 0.05). The cause of idiopathic exophthalmos is compression of orbital contents by the medial wall expansion. The force of expansion of medial wall also influences the zygoma according to Wolff law.