The typical Mongoloid eye differs from the Occidental one in puffiness of the upper eyelid and narrowness of the palpebral fissure. We surmised that an anatomical structure might exist in the preaponeurotic fat space, which determines the features of the Mongoloid eye. The upper eyelids of 720 Japanese patients and eight Japanese cadavers were macroscopically investigated, and sagittal slices of the central upper eyelid of one patient and nine cadavers were microscopically analysed. Another, lower-positioned transverse ligament different from the higher-positioned Whitnall's one was identified just above the fusional line between the orbital septum and the levator aponeurosis in almost every upper eyelid. A robust ligamentous structure was observed in 331 patients (46%) with puffier eyelids and a narrower palpebral fissure, a diaphanous retinaculum in 381 patients (53%) and no transverse ligamentous tissue in eight patients (1%). Histological evaluation demonstrated that this ligament was composed of tight bundles of collagen fibres containing few elastic fibres. Examination of the preaponeurotic fat space indicated that the superficial expansion of the levator aponeurosis turned up around this transverse ligament to become the orbital septum. When the levator muscle contracts, the deep expansion of the levator aponeurosis retracts the tarsus, the middle expansion retracts the pretarsal orbicularis oculi muscle and skin, and the superficial expansion retracts the preaponeurotic fat against this ligament. This structure thus determines the low position of the preaponeurotic fat and restricts the vertical width of the palpebral fissure, causing the features of the Mongoloid eye.