PURPOSE:: To clarify the causative factor of Asian double eyelid. DESIGN:: Experimental anatomic study. PARTICIPANTS:: Twenty-six upper eyelids (13 right and 13 left) from 17 Japanese cadavers (9 males and 8 females, mean age at death: 73.1 years). METHODS: The specimens, obtained from the central part of the upper eyelids, were dehydrated, embedded in paraffin, cut into 7-μm thick slices and stained with Masson trichrome. Statistical analysis was based on the Mann-Whitney U test. Statistical significance was defined as p < 0.05. MAIN OUTCOME MEASURES: Orbicularis oculi muscle thickness and shape, with or without the levator extension, orbicularis oculi muscle bundle spacing, thickness of upper eyelid skin and subcutaneous tissue, fusional site between the levator aponeurosis and the orbital septum, with or without inferior drooping of fat tissue. RESULTS: The orbicularis muscle was thinner at the skin crease of a double eyelid than at 10 mm from the eyelid margin in the single eyelid group (p = 0.029). In 3 specimens the skin crease of double eyelid was at the tip of the bending orbicularis muscle, which was thick. Although the skin crease in the region of a double eyelid was thinner than in other parts of the skin, excluding the simple crease regions, other outcome measures were not shown as definite causative factors in creating the Asian double eyelid. CONCLUSIONS: The thickness of orbicularis oculi muscle or its bending shape, and the thickness of the skin at the skin crease, are major causative factors in Asian double eyelid formation.