The presence of functioning vascular anastomoses within the monochorionic placentation of a monozygotic twin pregnancy is responsible for the twin-twin transfusion syndrome. This is a heterogeneous disorder with a wide spectrum of severity. In the most severe form it results in the oligohydramnios-polyhydramnios sequence with a marked disparity in intertwin biometry and amniotic fluid volume. Without therapy, the perinatal mortality of severe twin-twin transfusion syndrome is 60-100%. Several therapeutic interventions have been attempted with therapeutic amniocentesis producing the most consistent improvement in fetal survival rates. The outcome of 10 cases of severe twin-twin transfusion syndrome, presenting as the oligohydramnios-polyhydramnios sequence, is presented. Gestational age at presentation ranged from 20-34 weeks. Three fetuses were hydropic at initial presentation and of these only 1 survived. All cases were treated with serial aggressive therapeutic amniocentesis, the number of procedures ranging from 1-9/patient. The volume of amniotic fluid removed ranged from 3,200-14,000 mL. Gestation was prolonged a mean of 46 days (range 1-106 days). The perinatal survival rate was 65% (13/20 infants). Preterm premature rupture of membranes and secondary preterm birth complicated 1 pregnancy treated with therapeutic amniocentesis. Short-term morbidity in the liveborn infants was predominantly secondary to prematurity and renal cortical necrosis. Severe twin-twin transfusion syndrome is the most common aetiology of the oligohydramnios-polyhydramnios sequence. Therapeutic amniocentesis offers an intervention that may reduce the high perinatal loss rate previously observed with this disorder.