ABO incompatibility is a common haematological problem affecting the newborn. The haemolysis is widely accepted to follow a relatively benign course rarely causing the escalating levels of hyperbilirubinaemia and significant anaemia associated with Rh haemolytic disease of the newborn. Case reports of fetal hydrops secondary to ABO incompatibility are particularly rare. We describe two cases, first that of a twin pregnancy with both fetuses developing severe anaemia at 20 weeks gestation, and then a second case of a preterm baby demonstrating aggressive haemolysis and anaemia within hours of delivery. Both mothers were of black Africian origin and both were identified to have elevated titres of IgG anti-B antibodies.