Amniocentesis and drainage were performed in 19 twin pregnancies initially seen at less than 28 weeks' gestation with acute polyhydramnios caused by the twin-twin transfusion syndrome. The procedure was performed an average of three times for each patient (range one to six), with the removal of a mean volume of amniotic fluid of 2600 ml on each occasion (range 500 to 6700 ml). The interval between initial examination and delivery averaged 7 weeks (range 1 day to 18 weeks); 14 (37%) of the 38 babies survived the neonatal period. In the pregnancies with poor outcomes, the intertwin disparity in fetal size and the mean volume of amniotic fluid drained at each amniocentesis were greater than in the pregnancies in which one or both twins survived.