Background: Therapeutic amniocentesis has been accepted widely as a safe and efficacious way to treat the polyhydramnios-oligohydramnios sequence associated with twin-twin transfusion syndrome. Case: A 28-year-old woman, gravida 2, para 1, diagnosed with twin-twin transfusion syndrome at 28 weeks' gestation was treated with serial amniocenteses. The dividing membrane was ruptured inadvertently during therapeutic amniocentesis, with subsequent complete disruption of the amniotic membrane. Iatrogenic monoamniotic twins with cord entanglement and knotting resulted. Conclusion: Creation of monoamniotic twins by disruption of the dividing membrane can be a complication of therapeutic amniocentesis for twin-twin transfusion syndrome. Such disruption may result in the same morbidity and mortality that are seen in naturally occurring monoamniotic twins.