Hydatidiform moles are known to pose an extremely high risk of severe early-onset preeclampsia if left untreated. TNF superfamily cytokine, LIGHT has recently been reported to contribute to pathophysiology of preeclampsia. The present study aimed to investigate the involvement of LIGHT in hydatidiform moles. We measured the serum levels of LIGHT and sFlt-1 by ELISA in 17 women with complete hydatidiform mole (HM) and 20 gestational-age-matched normal pregnant women (control). As a result, the serum LIGHT levels were significantly higher in HM as compared with those in control (69.9 ± 9.6 pg/ml vs 25.4 ± 5.3 pg/ml, p = 0.0001) and the serum levels of LIGHT were significantly positively correlated with those of sFlt-1 in HM (r = 0.68, p = 0.0029). Immunohistochemical analysis revealed that the expression levels of LIGHT were increased in HM placentas as compared with controls, and LIGHT and sFlt-1 were co-localized in the trophoblast cells of HM. In vitro studies using primary syncytiotrophoblast cells demonstrated that LIGHT directly induced sFlt-1 expression in trophoblast cells. Our results indicated that elevated LIGHT in the trophoblast cells of hydatidiform mole induces sFlt-1, which might underlie the pathogenic mechanism of early-onset preeclampsia developing secondary to molar pregnancies.