We examined the impact of the presence of one anomalous fetus in a twin pregnancy on perinatal outcomes in the normal cotwin. Perinatal outcomes in cotwins with an anomalous twin and cotwins without an anomalous twin were compared using data from the 1995 to 1997 United States Matched Multiple Births dataset. The two groups were matched by maternal age, parity, birth order, gender, and sex concordance (1:4 matching). The risks of preterm birth, low birth weight, small-for-gestational-age birth, fetal distress, the use of assisted ventilation, low Apgar score, fetal death, neonatal death, and infant death in the 3307 normal cotwins with a twin affected by structural anomalies were significantly higher than those of the 12,813 matched cotwins without an anomalous twin. The presence of one structurally anomalous fetus in a twin pregnancy increases the risks of adverse perinatal outcomes in the cotwin without a fetal anomaly.