We present the first case of a monochorionic twin pregnancy in which sudden hematologic changes occurred as a complication of the amnioreduction procedure for twin-twin transfusion syndrome (TTTS). At 33 weeks of gestation, 4 days after the amnioreduction, the recipient developed severe anemia while the donor developed severe polycythemia. Postnatal placental examination revealed several arteriovenous and venoarterial anastomoses, a pale placental mass of the recipient and a congested and plethoric placental mass of the donor. We speculate on the pathophysiologic changes and potential deleterious effects provoked by the decompressive amnioreduction. Decompression of the placenta and anastomoses after the amnioreduction may have led to an acute blood shift from recipient to donor (thus also a reversal of feto-fetal transfusion), resulting in anemia in the recipient and polycythemia in the donor twin. In the past 15 years, 13 TTTS cases with late presentation were treated with amnioreduction. This is the first time we encountered this severe complication, yielding an incidence of 8%. Although the optimal treatment in TTTS with late presentation is not known, perinatologists should be aware that treatment with amnioreduction can lead to sudden hematologic changes.