The perinatal mortality among twin pregnancies by zygosity ws assessed. Of the 11,451 births, 130 twins were born at Yong Dong Severance Hospital between Apr. 1983 and Dec. 1993 including 44 cases of dizygotic twins, 42 cases of monozygotic twins (40 cases of diamniotic monochorionic twins +2 cases of monoamniotic monochorionic twins) and 44 cases of unknown zygotic twins. We may allow determination of zygosity in twins by prenatal ultrasonic evaluation of dividing membrance, sex-discordance and postpartum placental examination. The incedence of twins remained 1.13% of all pregnancies and monozygotic twins occured higher preinatal death than that of dizygotic twins because of their communicating placental circulation and its associated lethal anomalies. Eary prenatal ultrasonic evaluation of zygosity should be emphasized as well as recognition of twin pregnancy.