Twin anaemia-polycythaemia sequence (TAPS) is a rare form of feto-fetal transfusion between monochorionic twins. It occurs spontaneously or after laser surgery for twin-twin transfusion syndrome. TAPS is characterized by a large inter-twin haemoglobin difference and can be detected both ante- and postnatally. This is a case report of TAPS, detected antenatally by a routine scanning of monochorionic twins in gestational week 29.