Background: In monozygotic twin pregnancies, there are placental vascular communications between the two fetuses.  In 15% of such pregnancies, there is an imbalance in net blood flow between the twins, resulting in the twin-twin transfusion syndrome.  The recipient twin may have severe hydramnios during the second trimester of pregnancy, and there is a high risk of perinatal death and cerebral palsy in survivors.  This condition can now be treated by endoscopic coagulation of the vascular anastomoses responsible for fetofetal transfusion with a neodymium:yttrium-aluminum-garnet laser.  Methods: We performed intrauterine surgery in 45 pregnant women carrying twins at 15 to 28 weeks of gestation (median, 21); in each case there was severe hydramnios in one fetus due to the twin-twin transfusion syndrome.  With the use of local anesthesia and continuous ultrasound visualization, a rigid fetoscope 2 mm in diameter, housed in a 2.7-mm cannula, was introduced transabdominally into the amniotic cavity of the recipient twin.  A systematic search was made for all vessels approaching or crossing the membrane between the twins, and these were coagulated with a neodymium:yttrium-aluminum-garnet laser by means of a fiber in the side arm of the cannula.  Results: Coagulation of the communicating vessels was successful in all cases.  The total number of fetuses who survived to delivery was 48 (53%), and the number of pregnancies with at least one survivor was 32 (71%).  Among the live-born infants, the median gestational age at delivery was 35 weeks (range, 25-40), and the median birth weight was 2,098 g (range, 550-4,252).  The median interval between the endoscopic laser procedure and delivery was 14 weeks (range, 0-21).  All the survivors were developing normally at a median age of 12 months (range, 2-24).  Conclusions: Our preliminary experience suggests that the twin-twin transfusion syndrome can be treated effectively by endoscopic laser coagulation of the communicating placental vessels.