Twin embolization syndrome (TES) is a complication of monozygotic twinning following in utero demise of the co-twin. Passage of thromboplastic material into the circulation of the surviving twin results in ischemic structural defects of the central nervous system, gastrointestinal tract, and genitourinary system. In six cases of this rare abnormality, sonographically normal fetuses subsequently demonstrated ventriculomegaly, porencephaly, cerebral atrophy, or microcephaly following demise of the co-twin. Extracranial abnormalities included small bowel atresia (two cases) and renal cortical necrosis (one case). Surviving twins all had neurodevelopmental delay at follow-up. Two fetuses were aborted because of sonographically apparent, progressively severe brain abnormality. The lone survivor of a dizygotic triplet gestation was normal, despite sonographic findings of TES in the co-triplets. Because of poor outcome, prenatal sonographic diagnosis of TES may influence antenatal or neonatal management of the surviving monozygous twin. Recognition of TES may permit accurate postnatal counseling of prognosis and recurrence risks.