The prenatal histories, clinical courses, and neuroradiographic studies of 8 infants who had survived the in utero demise of a homozygous co-twin were reviewed. Three distinct modes of clinical presentation were found: (1) severe neonatal encephalopathy with seizures; (2) a more benign neonatal course with onset of seizures and profound developmental disabilities within the first 6 months of age; (3) late infantile presentation with seizures. Only the third group had milder outcomes. Neuroradiographic studies demonstrated two pathologic patterns: varying degrees of periventricular white matter infarction with migrational abnormalities observed with earlier demise of the co-twin, and multicystic encephalomalacia observed when demise occurred at or near term. Pathophysiology is uncertain and most likely multifactorial. Exsanguination injury to the survivor can occur acutely following co-twin demise, so urgent delivery may be appropriate at or near term.