We report a pregnancy complicated by anti-B isoimmunization that resulted in fetal ascites, anemia, hepatomegaly, and polyhydramnios. A previous pregnancy in the same patient was complicated by neonatal ABO incompatibility. A review of the obstetric literature suggests that ABO incompatibility may cause severe fetal anemia, especially in patients with type O blood or a previous history of ABO incompatibility of the newborn.