Antenatal sonographic diagnosis of twin–twin transfusion syndrome (TTTS) is based on findings of a twin oligo-polyhydramnios sequence (TOPS) observed in monochorionic twin fetuses. However, TTTS can develop without a significant characteristic intertwin discordance in the amniotic fluid volumes. We report an uncommon form of TTTS without TOPS showing severe anemia in one twin and polycythemia in the other. Based on sonographic findings, it is considered that the recipient twin became the donor later in gestation, and vice versa. It is concluded therefore that even in the absence of TOPS, the possibility of severe TTTS with a suspected reversal of donor-recipient phenotypes during pregnancy should not be excluded, and serial Doppler studies including the measurement of the middle cerebral artery peak systolic velocity should be routinely performed even in seemingly uncomplicated monochorionic twin fetuses without TOPS.